Sanford guide is updated every year, and the Johns Hopkins antibiotics guide is also a good choice, though be aware that Hopkins tends to favor European guidelines. If you are thinking peds, The Red Book is the book, and has a monograph on most organisms, etc. If you can't get ahold of those, the IDSA website, as mentioned above, has at least a link to most of their guidelines, and most school libraries should carry their journals, ID and Clinical ID, which are great.