I would recommend "Micro Made Ridiculously Simple"-- it covers topics in sufficient depth and is extremely easy to read. The mnemonics are great and the sections on antibiotics are particularly good. I used only my class' notesets and this book to study from and did well in the class and on the micro section of the step 1. Good luck!
I agree, Micro Made Ridiculously Simple along with First Aid for USMLE, will take care of all your micro needs. In the beginning I went with Micro & Immuno (Levinson/Jawetz), but found it too detail and difficult to sort/memorize. However MMRS is a really funny book and the diagrams stick in you head more readily. Your best bet would be to browse this book, it should be in your med school library, or just ask some of your classmates. Alright, later.