You're more likely to lose a part then break one. Firing pins, cam pins and firing pin retaining pins are usually what my soldiers manage to lose performing maintenance in the field. I'd recommend one of each if your that worried about spare parts.
As for books, FM 23-9 dtd July 1989 covers the M16A1 and A2 rifles. It has a lot of information on zeroing, ballistics etc. Check the local gunshows, usually someone has them for sale for about $5.00. Also get a copy of TM 9-1005-319-23&P Unit and Direct Support Maintenance Manual for the M16A2 and M4 Carbine. It has exploded views and detailed instructions on assembly and disassembly of your rifle. For weapons handling I'd recommend "The Urban Carbine, Part of the Solution" by Jim Crews. For the history and development of the AR15/M16 "The Black Rifle" by Edward C Ezell.