There are a number of books on differential diagnoses. Here are two that I have seen students use:
A Pocket Manual of Differential Diagnosis by Adler
Handbook of Differential Diagnosis in Internal Medicine: Medical Book of Lists by Greenberger
Of course, as the other posters mentioned, if you can get it for free, why not? If you have access to uptodate or emedicine, both do have info on differential diagnoses.
Of course, as you know, the differential diagnosis is only the first step and the books listed above will not provide you with guidance. That is, you still have to determine which one of the conditions in the differential is the one that explains your patient's clinical presentation. There are books that will help you with that as well. Feel free to send me a PM if you have any other questions. Good luck with those differentials!
Samir Desai, MD
Assistant Professor of Medicine
Baylor College of Medicine