"Crush Step 2" is an easy book to read. I read this twice before taking the test. I would say it has more detail than "First Aid for Step 2 CK", yet I read both of them. I thought they complemented each other well.
"NMS Review for USMLE Step 2 CK" provided a 1000 practice questions. I was able to finish the entire book. Its explanations provide a lot of background. Sometimes explanations repeat key points several times over different questions. I did all the questions once and I think it would be pretty hard to do the questions multiple times. Reading and underlining the explanations was probably where I learned the most for Step 2 CK.
"Boards and Wards" is a good pocketbook for clinical rotations as a medical student. It's also a thorough review book for Step 2 CK. I would say it's more detailed than "Crush Step 2" or "First Aid for Step 2 CK". I was able to read this 1-2 times.
"First Aid for Step 2 CK" provided good tables for the test. I read this book twice, yet I thought sometimes it provided too little detail in some areas. Reading multiple books will help the most.
I still think you need access to at least a good medical textbook, either "Harrison's" or "CMDT". Access to Access Medicine is great also. Sometimes the explanations on practice questions will need more background information.
USMLEWorld.com - Although many people found USMLE World to be helpful for Step 2 CK, I couldn't use it for more than 100 questions. Maybe it was because I was early in my studying or I thought the questions were too esoteric. The explanations are very detailed, and reading that much on a computer screen is not a good way for me to learn. There's about 2000 questions available.
NBME Practice Tests - The practice tests are worth the money spent on them, around $40 each. The analysis of your performance on different test areas is helpful early or even late in your studying. Their projected score was pretty accurate also.
Creating your own questions helps a lot. Group studying works.