Just one person's opinion:
First Aid to Step 2 CK - Flipped through it. Medicine sections seem poorly organized and woefully inadequate. I strongly prefer Step up to MEDICINE if you have the time -- a great book that reads smoothly. However, I will use First Aid to supplement the non-Medicine areas (the OB/GYN section in FA2CK seems particularly thorough and well-organized; Pediatrics; Psych)
Step Up to Step 2 - an alternate "all in one" package. The medicine portion is very much compressed, and since Medicine is a bulk of the Step 2 exam, I would go ahead and read Step Up to MEDICINE if you can. But the other sections look decent.
Step 2 Secrets - Great to read through at the end to hammer home fine points (which is what I'm planning to do), but not sufficient as a primary text.
Boards and Wards - Another good book if you have time, but again more of a condensed review source than a "central text."
None of this should precede USMLEWorld. I think Kaplan is garbage and far too nit-picky (this opinion formed after completing approximately 350 questions), but I suppose it can't hurt. NMS question book was also good (again, did about 250 questions before stopping) but also intermittently picky and hit-or-miss. USMLEWorld is money all the way.
My strategy was to complete USMLEWorld first, which I did recently, THEN do my reading -- as I'm reading, my mind is instinctively honing in on the facts that I had initially missed in World and then came across in the question explanations -- then building it into a framework as I study and annotate. This requires time, but I strongly recommend it.