In my experience with medical education, I have yet to find a resource as versatile as Step Up to Medicine. I first purchased this text as a third year medical student on my internal medicine clerkship and was instantly impressed with both the level of detail present and the organized flow that takes you from how a disease presents through how it is diagnosed and managed.
Needless to say, I gave this book a lot of credit for how well I did on my IM shelf, and when several of my classmates told me how much medicine was on the surgery shelf, I used it again to obtain a very high score. This resource later came in handy for studying for my family medicine (the ambulatory and infectious diseases chapters are very high yield) and neurology shelf exams. Naturally, given the positive results that I had throughout third year, it felt like a no-brainer utilizing Step Up to Medicine again to prepare for Step 2 CK.