the books I have read are :
Careers in osteopathic medicine by Terrance Sacks
The DOs: Osteopathic Medicine in America by Norman Gevitz (really good book on history)
and
Osteopathic Medicine : A reformation in Progress by Gallagher and HUmphrey
last one is a good book too but I dont think its worth the money.
Id say get the gevitz book.
also the aacom.org website