I'm in the midst of my Medicine Clerkship right now. Here are the books I purchased and how useful I felt they were:
1. CMDT 2004 - I think, for medical students, it is the best book out there that serves as a comprehensive guide. As soon as you are assigned a new patient, crack this baby open to get the full run-down on the disease, its management, symptoms, key findings on physical diagnosis, etc. Texts like Harrison's are more comprehensive, but not as user friendly.
2. First Aid for the Wards - Quite a neat little book, has many things in a small package. It has an extremely high-yield section for each major clerkship that you can study weeks or months in advance. In addition, it tells you how each clerkship works and how to excel. Finally it has a listing of review books, PDA programs, and textbooks and their ratings for each clerkship.
3. Blueprints Medicine - The quintessential review book for the Medicine Shelf. Some people prefer First Aid for the Medicine Clerkship, but I personally prefer the Blueprints format.
4. Saint-Frances Guide to Inpatient Medicine - This book is probably a little too detailed for the medical student. Very good for your intern year however.
5. Griffith's 5 Minute Clinical Consult (PDA) - An outstanding program to have on the wards to give you a quick rundown on a disease, its symptoms, medications, and prognosis.
6. Epocrates Rx Pro (PDA) - Possibly the best pharmacological guide ever. This program has saved my butt countless times already. It has a full index by both generic and trade names for drugs with complete Adult/Peds dosing information, side effects, mechanism of action . . . hell it even has the manufacturer and cost! It has also made my intern's life a lot easier since I can give them drug doses within seconds.