If you are going into internal medicine, EM, or surgery:
If you don't already have them, either Mosby's "The Care of the Medical Patient" or the Washington Manual version are great general medicine handbooks. Fits nicely in a whitecoat pocket as well. I am a surgery resident, and at the beginning of my intern year, I was treating things like hyperosmolar coma- ill-appearing, middle-aged man with a h/o kidney transplant presents to our acute care center, diabetic, BG=800, confused, urinating all over himself, and I was *completely* alone to deal with it. As a brand new intern, I wanted to pee all over myself as well. But I pulled out the Mosby's guide, made sure I considered all of the steps in the "Therapy" section, and by the time the attending came to round, the patient was alert, appropriately oriented, and his BG was under control. He did very well and went home a few days later.
For surgery, I personally like the Mont Reid handbook- you can read up on lots of things you see on the wards while you are actually on the ward- this also is pocket-sized.
Advanced surgical recall is great, as well as the Marino ICU book, though they are too big for the pocket so you have to stuff those puppies in a backpack.
And don't forget about the good ol' ACLS handbook.
good luck