Good Books
There are many good books on the M14 out there. One of the best for more information than you ever thought you needed to know is Lee Emerson's M14 RIFLE HISTORY AND DEVELOPMENT. This is only available through
www.lulu.com.
Other great books are R. Blake Stevens US RIFLE M14 from John Garand to the M21 from Collector Grade Publications and The Last Steel Warrior US M14 RIFLE, by Frank Iannamico.
These three books are a wealth of information on the development of the M14 and all the details that go along with it.
As far as detailed inspection and maintenance I'd recommend The US .30 Caliber Gas Operated Service Rifles - A Shop Manual, by Jerry Kuhnhausen. This is practically a bible for armorers.
There are several other smaller and cheaper books available, but much of the information contained in them should be taken with a grain of salt.