If you are now having a flashback to your university days of standing in long lines and forking over huge sums of cash, relax. These textbooks are free online.
Todar's Online Textbook of Bacteriology - http://www.textbookofbacteriology.net/index.html
Medical Microbiology, 4th Edition by Samuel Baron - http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mmed
Microbiology and Immunology Online from University of South Carolina Medical School - http://pathmicro.med.sc.edu/book/welcome.htm