For history and research, the Black Rifle books are the best. Both are entirely independent (II isn't an updated version of I). These books are about 90% pictures if that's what you're looking for:
"The Black Rifle" by Stevens/Ezell: History and development up to 1987. A lot of good info for retro builds.
"Black Rifle II" by Bartocci: Continuation of development from 1987 on. Good stuff for SPR, recce type builds and M4 stuff. Also a good section about forge codes and markings that goes all the way back. Covers the Colt Sporter rifles as well.
For general information, the "AR15 Sourcebook" by Duncan Long isn't too bad. It covers a lot of the history, but also how to assemble/disassembly the entire rifle. There is a newer 2nd edition, but it is still getting pretty dated concerning accessories, etc. There are probably better general info books out there nowadays, but I don't have any.
For tactics, I like "The Book of Two Guns" by Tiger McKee, but the ones mentioned above are good as well.