Sorry in advance if this has been answered before (I did some digging and found nothing in the help section), but how do you obtain books to raise your pet's intellegence levels?

In Main Shops there is a store called Book Nook. Buy books there and read them to your pet.

Ah! Thanks :)

Subetalodge lists all of the books available. user shop search will show Main shops, Pawn shop, and user shop prices when available.

there are many secondary sources for books, so you can find music books in the Music Shop, books about books in Alexander's Quest Shop, prize pools for plots and site events, and right now Melody is giving a few out as presents for Luminaire.

You can pretty much get books everywhere. Main Shops. User Shops. Donation Presents/Collections. Token Shop. Quest Shops. Pawn Shop. Pretty much everywhere. :p

If you're looking for the cheapest books, your best bet is going to be the main shops, especially, Book Nook. You might also want to do a search on Subeta Lodge because you can search books in a specific price range.