Agreed that NMS is a good book to read for a 3rd year rotation. You will get lots of suggestions from residents who tell you the names of bigger books that are great to read (Baby Miller, Morgan & Mikhael, MGH Handbook, Secrets, etc). However, even the simplest of those is like 200 dense pages. You will not get through them during your 3rd year rotation.
NMS is not an exciting or a detailed book, but it's tiny and quick to get through in just an hour or so. You have a good chance of actually finishing it and getting a good overview. I read it for my 3rd year rotation and it did me well. For my aways, I did graduate to bigger books (Baby Miller, MGH Handbook) -- but the foundation I'd gotten from NMS was what made me able to read and comprehend specific topics in the bigger books since there is no way you will have enough time to actually read the entire Baby Miller or MGH handbook as a medical student.