I could write a list pages long, but I'll try to hit the highlights:
History - none better than Gevitz' The DOs; Yep he's faculty here at OUCOM
Physiologic theory - Read any of the works of I.M. Korr. There is also a great series by Chaitow (british osteopath) that does a wonderful job with some of the new research. He has a book on palpation skills and almost every manipulative technique, including cranial.
Anatomy related - Anatomy trains by Moore is spectacular. It will change the way you approach your patients. Also, The Endless Web by Shultz/Feitis is great for fascial anatomy, which is often crapped on by "standard" anatomy texts.
Book to definitely spend money on first year - Osteopathic approach to Diagnosis and Treatment by DiGiovanna. This was my bible last year. Tons of pictures and step by step instruction on many techniques.
Interesting read - Osteopathic approach to children by Carreiro. This book is heavy on anatomy and phys and low on treatment. Good background text, especially for those working with peds.
Foundations of Osteopathic Medicine is the gold standard, but I have to admit, it's one of the few OMM books I don't own. It's really cumbersome and often difficult to read through. But, no other book has the encyclopedic value of Foundations. The library should have about ten copies...save your dough.
There is a really excellent manual available for Still Technique, but the author's name escapes me. Sorry. It discusses some of the origins and evolution of AT Still's treatments.
If you want to learn about how your viscera acts on your musculoskeletal system and vice versa read anything by Myron Beal.
You know what...this is taking to long... just go to this website and buy all the books...then read them. There will be a fifty point quiz after class.
Have fun!
http://www.academyofosteopathy.org/pub.cfm?page=catalog