The study of soldiering, especially tactics, is very important.  We have a 4 thousand year recorded history of war, it would behoove you to study it. Besides, it's interesting stuff.  I have included a list of books that I have found useful in the study of military history, revolutionary warfare, tactics and skills.  The list is long, but far from complete.  They are not equally useful so I have added comments that might help you pick some or prioritize.

The Ranger Handbook
by the US Army Infantry Board
This is perhaps the single greatest book on small unit tactics out there.  The sections on fire support and Army aviation are about useless to us.  But there are excellent sections on operation orders, patrolling, movement, battle drills etc..  Look out for the version you get.  Some our a nice handy pocket size but might approach unreadable (due to the fact that it has been copied and downsized.)

Book of two Guns
By Tiger McKee
Getting instructional information on practical shooting from a book is pretty hard.  It is a “hands on thing” and there is no way of escaping this.  But, Tiger McKee makes a good job of it here.  The book is basically his observations from taking classes from and teaching practical rifle and pistol marksmanship for many years.  There is a lot of interesting “mindset” and psychology of combat information up front that I really liked.  Then the book gets down to business and covers various shooting, malfunction and tactical drills.  While the book is mostly about the 1911 pistol and the AR carbine most of the skills will apply to any rifle/pistol combo. If you read and then practiced what he says you will learn much faster than just practicing on your own.  Some people might not like the format, basically the book looks like a classroom notebook, hand written and all, but I think it’s pretty clever. 

Light Infantry Tactics: For Small Teams
by Christopher , E. Larsen (Author)
This book maybe replaces the venerable Ranger Handbook for the Citizen Soldier. This book does a very good job at outlining various dismounted infantry skills.  It does it in a way that escapes the pitfalls of military FMs… i.e. to much stuff that doesn’t apply to the civilian, poor print quality etc.  It is in an easy to read format that makes the information accessible and understandable.  Illustrations are timely and helpful.  It addresses individual skills, leadership skills and group skills.  Individual skills cover things like camouflage and hand signals, leadership skills cover things like patrol planning and op orders, group skills are communications and various tactics. I would like to see a small, pocket sized field version, but that is a trade off for readability.

Total Resistance: Swiss Army Guide to Guerrilla Warfare and Underground Operations
By Major H. Von Dach Bern
The Swiss long planned to be overrun in the next major European conflict.  This was the manual they came up with, a guide for citizens to continue the fight against a totalitarian occupying force.  It has a lot of stuff you will have a hard time finding elsewhere.  Including a lot of stuff on tactics and logistics specific to resistance movements, supplies and setting up a secure resistance organization.

Other Military Field Manuals
There are a whole lot of these out there, many of them can be found online for free (in downloadable PDF format) you might want to check "Global Security" in the links page for this, or on Amazon. The problem is that most of them assume that you will have the resources of the Army writing them.  It takes a good bit of sifting to figure out what is useful for you.  I have listed a few that I find helpful.

FM 5-31 Boobytraps
– Includes a lot of pre-made, yet unavailable to you stuff.

FM 21-75 Combat Skills of the Soldier
–Very good for beginners (including all of us)

FM 7-8 Infantry Rifle Platoon and Squad
–Similar to Ranger Handbook

FM 90-10-1 Infantryman’s Guide to Combat in Built Up Areas (MOUT)
-Good info in urban high intensity combat

There are also good Canadian FMs out there.  Search for Canadian Field Manual “Ambush,” “Fieldcraft” and “Patrolling.” Each is a separate manual.  I find that they provide a useful non-American viepoint.

Leadership and Training for the Fight
by Paul Howe
Howe is a retired Master Sergeant with many years in the military’s special operations community.  You might remember him as “Sanderson” in the movie Blackhawk Down.  This book is his bible on how to prepare for serious conflict.  It gave me more insight on preparing for combat than any book I have read.  Some of the stuff will be useless for the citizen soldier (particularly the stuff on selection), but it is well worth the read.

SAS Survival Guide Handbook
by John 'lofty' Wiseman
Considered by many to be the end all of survival manuals.  It covers valuable (non-combat) survival skills in a clear and concise way.  You can’t fight if you die of exposure folks!  If you can find the smaller version I would get it… the one I have is a bit large to fit in a pack.

How to Survive Anything, Anywhere
by Chris McNab
Chris McNab should not be confused with Andy McNab the SAS vet and author of several books.  Chris primarily rights books on military history, but also writes excellent books on survival and combat skills.  This book might be a slight bit better than the SAS Survival Manual listed above, if you are only going to get one.  What it has, that the SAS manual doesn’t, is insight into how to survive more manmade disasters, and “street” survival skills.

The Counter-Insurgency Manual
by Leroy Thompson
I’m not sure what Thompson’s credentials are but this book brings together an eclectic number of topics.  There is a review of the works of Mao, Che Guevara and Marighella that will alow the beginning student of guerilla warfare to skip them (for now.)  There are also interesting topics that will not apply to the citizen soldier.  There are good explanations of ambush and counter ambush techniques and information on small unit patrolling, with diagram.   It also brings in examples from just about every low intensity conflict fought since the 1950s, with a lot of good pictures.  What this book lacked is a good editor, it’s like a high school student wrote it.

Where There Is No Doctor: A Village Health Care Handbook
by David Werner
This is a comprehensive book on medical care beyond first aid.  Most civilian medical info is geared towards “get them to a doctor as fast as possible.”  As the title suggests this is what you do when you can’t.   There is also a “Where there is no dentist,” out there.  Probably as useful, but I haven’t read it.

Blackjack-33
by James Donahue
This is a story of how unconventional warfare, how it should be done.  It is the personal history of the author as he served with the Mobile Guerilla Force in Vietnam.  While short on actual skills you will learn, or equipment lists etc.  It offers valuable insight in to what it takes to fight this kind of war.  I enjoyed it immensely.  This is the middle book of a trilogy, all are good but this is a decent place to start if you might not have time to read them all.

On Guerrilla Warfare
by Mao Tse-tung
No, I’m not a communist, (in fact I’m a dedicated ANTI-communist) but this is an interesting book on the Guerrilla war waged by the Red Chinese against the Japanese.  Most US Military manuals assume you are fighting within the constraints of the larger military machine, with all the advantages that gives.  You will more likely be fighting alone on the streets or in small bands and have none of these advantages.  It would do any armchair soldier to read something written by a man who actually did it.  This is available free online in several places.

Guerrilla Warfare
by Ernesto “Che” Guevara
Same as above, but with a more modern perspective and more interesting stories.  Che was “in the grass” more than Mao.  You can tune out his Communist/Socialist diatribes.  This is also available free online in several places.

Minimanual of the Urban Guerrilla
by Carlos Marighella
I hope you’ve heard of Mao and Che, but maybe not Carlos Marighella.  He was a revolutionary in Brazil.  Some of the types of operations outlined may seem distasteful to you, but reading about them and keeping them in your toolbox can’t hurt.  Still the same as above, but geared towards the “urban guerrilla.”  The tactics are better outlined and explained. 

Phantom Soldier: The Enemy's Answer to U.S. Firepower
by H. John Poole
Poole is a Marine Vietnam Veteran and serious writer on the things that are wrong with the US Military (and they seem to be listening.) In this book he examines several enemies of Americas past.  He focuses on the Vietnamese and Japan, but the gist is methods that they used to overcome America’s technological superiority.  It is well illustrated with diagrams of positions and tactics.  This is a truly handy book, and a must for every library on this sort of subject.

The Tiger's Way: A U.S. Private's Best Chance for Survival
by H. John Poole
Another Poole book.  This one advises ways to enhance military training to face the changing battlefield.  It explores the mistakes of the past and suggests ways to correct them.

Home Workshop Explosives
by “Uncle Fester”
Explosives manufacture and use is an important skill area.  Of course, it is also illegal for just about everyone.  I don’t suggest you do anything in here, but having this book set aside for a rainy day is a good idea.  If you don’t have a little chemistry background though, it might be next to useless.  Also. “Uncle Fester” is a nom de plume of a guy who (I think) would agree with my thoughts.  I don’t blame him for wanting to remain anonymous.  He has written other books on drug productions (something I don’t recommend.)  I think he writes to “stick it to the man.”  Not a bad thing in and of itself.  Anyway… explosives are

Improvise (d) Munitions Black Book volumes 1&2
by The US Army
These books achieve about the same thing as Uncle Fester’s book above.  But it has a more “from scratch” methodology.  By this I mean you probably won’t need a background in chemistry, but the end results probably won’t be quite as useful.  There are also pages on how to employ these things and copious diagrams.  Again, I suggest you only have this book as a reference.

Tactical Advantage: A Definitive Study Of Personal Small-Arms Tactics
by Gabriel Suarez
Gabriel Suarez has written a lot of books on tactical shooting.  I would suggest this book as a guide to in the house “CQB” tactics for the interested civilian.  Lots of good advice, with good pictures and diagrams to explain it.  I would ignore the stuff up front though.  His comments on mindset are way off the mark and he may be the cockiest writer I have ever read.

To Ride, Shoot Straight and Speak the Truth
By Jeff Cooper
Ex Marine Colonel Jeff Cooper is considered to be the father of modern practical shooting.  While the shooting we do now has evolved a great deal, it is worth it to read it from the beginning.  This book is a collection of Coopers articles.  Very few of them deal directly with shooting.  They are more “philosophical.” It is darn good reading though.  If you’re looking for direct shooting advice there are better books, some by Colonel Cooper.

Black Hawk Down: A Story of Modern War
by Mark Bowden
War is a horrific ugly thing.  This is even truer in the modern day and age.  Just to get away from thinking that it might be a merry good time I suggest you read this book.  It is of course the story of the Battle of Mogadishu where 100 or so Army SpecOps types fought thousands of local Militia in an African shit-hole.  It is in your face reality based on the eyewitness accounts of dozens of participants on both sides.  It is also a good example of what happens when well-trained and equipped warriors meet up against a savage undisciplined mob.