Just finished it and got one of the alternate endings (shields and serpents). Honestly what a wonderful game. The placement of the puzzles feels really well thought out, making me go "aha!" every few minutes as I found out a way to deal with something I saw 15 minutes ago. The shields and serpents puzzles were devious but it was such a rush when I finally figured them out (with help from the comments here).
I'm assuming there's another ending that has something to do with the secret form that you don't need to finish the bestiary, but I haven't been able to figure out where to use it. Any hints on that?