First Aid for the USMLE Step 2 CK This is one of the most commonly used review books for step 2 CK. While you shouldn’t use it as your only review book, it does a decent job of covering the basic topics you will need to know. We found it to not be comparable to First Aid for the Usmle Step 1, which is considered a gold standard for preparation, however, it is a decent option that will not break the bank.