I've read a lot of books recently.
Step 2 secrets is the best bang for your buck. Read it.
Step Up to Medicine is the best book if you have time -- the correlation between SU2M, World, and High-Yield topics is unreal. You need something else for OB, Peds, etc. but it's an amazing book.
Step Up to Step 2 is oversimplified in my opinion and really skimps, but some of the tables are good -- I just skimmed through it for a few hours. If you want an "all-in-one" book and don't want to spend too much time reading this is probably your best bet. I'm just the sort of person who likes to cover bases.
First Aid CK is garbage -- I just skimmed through it for a few hours. The medicine sections are particularly bad (use SU2M if you can) -- but the OB/GYN and Peds sections seemed better. I just didn't have time.
Boards and Wards is also a bit hard to follow and includes a lot of details that aren't particularly relevant, and the bullet style is harder to follow. I decided to skip it, but it's probably worth a read.
Master The Boards Step 2 is great because the other books skimp on management but this is where MTB shines; however, certain sections are really weak (Gynecology, Neurology). I think it's invaluable just because it emphasizes management which is a weak area for us.
MTB Step 3 fills in the management gaps nicely for weak parts of MTB2 (GYN, Peds) but often goes a bit overboard with dosing and management -- you should really be focusing on initial diagnosis and steps in management right now because that's what CK will test. So I just used MTB Step 3 to fill in Gyn and Neuro for a few hours and didn't fixate on it. There are higher yield sources elsewhere.
In the end, my final recommendations, and the sources I actually used to completion:
-USMLEWORLD
-Step 2 Secrets
-Step Up To Medicine
-Kaplan MTB Step 2 (great for management)
-Kaplan MTB Step 3 or another source for OB/GYN and Peds supplementation.