"Bestiary" is a translation of a medieval book about animals, originally written in Latin during the 13th century, somewhere in England. The main source for this bestiary was another bestiary, compiled during the 12th century, and also translated to English as "The Book of Beasts". Thus, the text of the two books are very similar. If you just want to buy one of them, I recommend "Bestiary". The reason? The illustrations are much better, and they are all in full color! Indeed, the illustrations are taken straight from the original manuscript, making "Bestiary" not just a translation of an ancient book, but also an example of medieval (Gothic) art.
We are used to a modern, scientific view of nature and animals. The Middle Ages saw things differently. Animals weren't seen as random products of blind, natural forces. They were created by God for the edification of the human race. Indeed, Adam named all animals in the Garden of Eden, each name reflecting their true character. Animals were not just brute beasts. They carried a moral message, directed to sinful humanity. They also carried a hidden, mystical meaning, which somehow paralleled the message of the Bible itself! All the world was seen as an enchanted, magical place, with each thing a symbol for deeper, moral or spiritual, realities.
This explains the rather strange style of "Bestiary". It's not just a collection of (often badly distorted) zoological facts. It's also a book of moral edification. The anonymous author often digresses from the "real" subject (the animals), and starts to preach Christian morality to his readers. That, too, was considered part of the subject. After a short description of the pig, the author attacks sinful gluttons and unclean heretics. The author also claims that coots take care of eagle chicks that have been rejected by their parents. The moral lesson? Animals are better than humans, who treat the children of strangers with out-most cruelty! The bestiary further claims that lions don't kill humans who prostrate before them, that they liberate captive humans, and that they never attack women. Once again, the lesson is clear: the lion has a Christian morality, while man too often hasn't. One of my favorite birds, the jackdaw, is apparently an apt symbol for chattering philosophers and heretics, not to mention gossipy and greedy men!
The mystical meaning of various animals is the most far-fetched aspect of the bestiary. All of nature somehow proved the Christian message of salvation. The virgin birth of Christ was "proven" by the claim that female vultures (!) gave birth without sexual intercourse. The resurrection was paralleled by the fiery death and re-birth of the Phoenix. The bestiary further claims that female lions give birth to stillborn cubs, but after three days, the male lion breaths life into them. This points to the resurrection of Jesus, and also to the Biblical passage "He couched as a lion, who shall raise him up?" (Genesis 49:9).
What particularly strikes a modern reader of "Bestiary" is the dismally low level of real zoological knowledge available during the 13th century. To a large extent, the bestiary was a collection of tall-tales. Indeed, many of them seem deliberately tailored to produce Christian parallels, such as the legend of lion cubs being "resurrected" on the third day. Of course, this fable-like quality makes "Bestiary" a very entertaining read. Thus, we learn that the beaver will bite off his own testicles if pursued by hunters, that a certain antelope in Africa can change its colors like a chameleon, that lions fear white roosters, or that barnacle geese grow from sea-weed! Some of the creatures in the bestiary are purely imaginery, such as unicorns, dragons, the manticore, and the eale (a deer with movable horns). Only the bat seems to be reasonably correctly described, without legendary or mystical accretions.
"Bestiary" is an entertaining read, an excellent work of art due to its full-color illustrations, and offers a fascinating look at the medieval world-view and mentality.
But if you want to know what's really shaking in the animal world, please buy a modern science book, LOL.