Squires is a good resource for an M4 rotation.
I also read Radiology Recall (a couple of students who also matched into Radiology read Radiology Secrets). I found the book useful for asking a few thoughtful questions during the rotation.
As a medical student on a general radiology rotation, the key is to present yourself as someone who is pleasant to work with and interested (but not a gunner) in learning. Try to get to know the residents and attendings, and find out from people the year ahead of you which attendings write strong letters of recommendation.