The Merck Veterinary Manual
According to the website "Merck is committed to bringing out the best in medicine. As part of that effort, Merck has created The Merck Manuals, a series of books for human and animal health care professionals and for the general public. As a service to the community, the content of The Manuals is available free of charge in enhanced online versions. The online versions are updated periodically with new information and contain illustrations and audio and video material not present in the print versions."