I really like the beastiary, and an element of it being incorporated into the knowledge system makes a lot of sense, once you are able to add more objectives to learning a skill besides just killing animals it will feel a lot better.
One suggestion you probably have already considered (or maybe even implemented) is unlocking crafting recipes through the Beastiary, it will make a ton of sense thematically, you start killing something like a spider, start collecting spider silk and spider fangs, and as you kill more you figure out how to effectively craft items that use these animal parts.
