Are books the only way to gain APs? And how many does a pet need to read before they gain one?

Books help to unlock AP yes. Every 16th Intel point will unlock 1 AP.

But if you go past 16 it's ok, when you read a book to your pet you will gain any missing AP. (say you boosted well over 50 Intel points, read one book and you will gain any missing AP from that.)

- Thank you! Are there other ways to gain AP, other than books? Or are books the only way to go?

All I know is books, sadly. That is all I can help with. :( I am sorry.

- Aww it's ok! You're a big help!! Guess I'll have to keep reading books to my pets then. It's getting tedious tbh.

you can still raise the pet's intelligence in other ways (stat boosters, random events, scratchcards, collecting pet income, etc.) - you just then have to read them a book to actually receive the AP they've earned. it's like a backlog; i could boost my pet's intelligance to 1600, then read them one single book, and they would gain all 100 AP at once.

You can also raise a pet's int by playing Mystic Spindles, but this works best before reading them a bunch of books since it costs more points to buy them int as they get more int.