If you want, you can read First Aid for the Wards before you start rotations, but there's really no need. You might want to go over Dubin's EKG book especially if you have Medicine first.
In terms of what to ask M3/M4 students at your school, try to find out how clinical evaluations work at your school (which attendings will be evaluating you - the attendings with whom who worked or the clerkship director?, what percent of your grade is based on the NBME shelf, etc).