Depending on how busy your rotation is, you may not have much time to read books. I use UWorld in tutor mode as my primary study tool - basically, I treat it as an interactive textbook - and supplement it by reading on my patients on UptoDate, watching OnlineMedEd for bird's view topic overview +/- reading textbooks/review books if I have time. Eg. I barely read any Step Up to Medicine during my very busy medicine rotation but did UWorld and OnlineMedEd + tried to learn as much as I could from my patients, and it worked just fine for me. YMMY though, and how well you do on shelves also seems to depend on whether you are a good standardized test taker and whether your basic science background is strong (believe it or not, it matters, especially with things like random questions about nerves or neurotransmitters on OBGYN shelf, I kid you not!)