Bergey's Manual of Systematic Bacteriology (initially deterministic bacteriology) is likely the most used reference source amongst microbiologists. The first edition came out in 1923 and has been continually added to and updated since that time. This extensive resource is divided into 5 volumes.
*note: if the text is clickable, we have access to e-versions. We also have it available in our reference collection in Tier 2 of Parks Library.
Volume 2: The Proteobacteria