Comparative Alien Physiology was a medical textbook written by Doctor Leonard McCoy. Considered a "classic" of the field, it was included in the medical library of USS Voyager.
In 2374, Harry Kim downloaded the book, along with Gray's Anatomy and other seminal medical literature into his EMH Replacement Program, in case The Doctor did not return from the Alpha Quadrant. (VOY: "Message in a Bottle")
In "Ashes to Ashes", Seven of Nine included time for study of this topic on the Borg children's activity schedule. While it was never expressly stated, this is likely the text they would have used.