I knew the list I made would come in handy. The location of Page 15 is hinted at after completing a certain tree's request. If you still require more hints after this I can tell you which tree in particular.
It involves the tree that wants the Lilyth. Once you can properly handle one and talk to it, I believe it tells you about something underneath it. If you didn't get the hint after showing it the Lilyth the tree then becomes the one that slaps your butt. The tree that gives you the Crusty Key is the tree you are looking for.