The be-all-and-end-all book...
I gotta go with Scott Duff's "The M14 Owner's Guide". Goes into history, (although not as in depth as a couple of others), variations, (incl the XM-25), how to inspect a potential used rifle purchase, and an incredible section on modifying a std. rifle to match conditions. This section alone is worth the cost...it really drives home the fact that YOU CAN'T DO THIS WORK YOURSELF!!! It takes a real pro with a whole range of experience to pull it off.