I love collection achievements, so I have three pets and each one I'm focusing on doing certain collections with like # of Books, # of Foods eaten, etc. The one that I'm focusing on for Books though, I've noticed there are certain books I just can't read to her. It doesn't show her as an option, but shows the other two. Is there a criteria for this that I'm just not aware of?

Have you checked to make sure you haven't already read those books to the pet before? They can only read each book once.

Well! I feel dumb, but you're absolutely right I missed this book entirely when going through the "books read" list for my girl. Thank you!