Costanzo Physiology and/or BRS Physiology
Costanzo Physiology is an excellent book; it has these great schematics and color figures that helps you understand all the little details of Physiology. It's written in a very simple straightforward way, so it's easy to read. It also has end of chapter open-end challenge questions. The downside: it is not a review book so it might take a bit longer to get through (it's about the same size as
Goljan RR but not as dense). If you have the time though it's totally worth it.
If you don't have the time,
Costanzo BRS physiology will do the trick; it's a review book so it's small and is written in bullet point format. Another benefit to this is that it has about 15-20 multiple choice questions at the end of each chapter, which really tests your understanding of what you just read.
I have both books, I am reading the big one and doing the end of chapter questions in both. When I get ready to seriously start Step 1 studying, I will switch to just
BRS physiology.
Hope that helps!
