“The Black Rifle” by R. Blake Stevens and Edward C. Ezell is certainly the definitive book on the development of the M-16. However, it doesn’t cover areas like safe handing, maintenance, and the shooting of the AR. Also, as Troy mentioned, it’s dated.
I suspect an 11 year old would find it pretty dreary.
For complete coverage, I’d recommend “The Complete AR-15/M16 Sourcebook” by Duncan Long. Admittedly, Long can be a little sloppy at times, but that is made up for by enormous amount of good information in this book. Be sure and look for the second edition (2002) of the book.
Other, shorter, but worthwhile books are:
“The AR-15 Complete Owner’s Guide”, by Walt Kuleck and Scott Duff. (I’ll agree with dcoke76 that this is a great beginner’s book for an AR owner.)
“The Complete Guide to AR-15 Accuracy”, by Derrick Martin and Barrett Tillman.
“The M16/AR15 Rifle, A Shooter’s and Collector’s Guide”, by Joe Poyer.