Comparative Alien Physiology was a medical textbook written by Leonard McCoy. It was of sufficient importance to be included in the medical library of the USS Voyager.
In 2374, Harry Kim downloaded the book, along with Gray's Anatomy and other seminal medical literature into his new EMH when The Doctor seemed unlikely to return from the Alpha Quadrant. Kim deemed it a "classic" medical text. (VOY: "Message in a Bottle")