At the time it was critizised for not telling the inside story but i thoroughly enjoyed Scott Freeman's "Midnight Riders: The story of the Allman Brothers Band" from 1995 with remained the only lenghty book on this subject for many years to come.

When it comes to telling the story in pictures "Les Brers, Kirk West’s photographic journey with The Brothers"
is a real must have.

Also take a look at this:

http://abbdiscography.duaneallman.info/index1.html

Skydog, by Randy Poe.

A Book of Tails - The Legendary Red Dog
No Saints No Saviors - Willie Perkins
Chuck Leavell - Between Rock and a Home Place
A Never Ending Groove - Johnny & Ann Sandlin

While all those are not 100% ABB-centric they all have the ABB as a central theme.