I'm still working on when I'm taking Step 2--I've only been back from the lab for 6 months, and I still have a couple core rotations to finish up. Probably going to take it in November--from what I hear, the programs that require Step 2 only require it for ranking and not to offer interviews, and fortunately I did well enough on Step 1 to get interviews at the places I want to go.
So far as books for the EM rotation, my usual study method is to get one good book and supplement with UpToDate--this has been very successful for me so far on shelf exams. Since I anticipate I'll be reading a lot more on my EM months (more time, plus it is far less of a chore for me to study EM than other specialities) I've upped it to two books, Emergency Medicine Secrets (by far the most-recommended book from what I've seen) and Case Files (which I've found fits my learning style well).