First Aid is not the essential book for Step II that it has become for Step I.
I like it. I don't think it's particularly useful for rotations - not enough detail. Many people prefer Secrets or Boards & Wards. A few years ago, Crush Step II was the standard on this forum, but now it is not very popular. In my opinion it is a good book for passing, but a poor book for getting a high score.
In my opinion, 99% of Step II preparation is in studying for your shelf exams, and doing USMLEWorld. Comprehensive review books aren't that useful.
For Step 2CS, the First Aid book is perfect and completely sufficient.
I'm from Japan, the term is roughly Japanese Spirit, Western Skill. It's sort of a late 19th century slogan, but I thought it appropriate to my background.