I've been upping my pet's Intelligence by reading books, but the results have been iffy on matching up. Last night, my pet had 1+ intel to books, but today after more books, it's now -1 to books, as though at least one or two books didn't give him an intelligence point. Is this a normal thing, or should I file a ticket to repair this?

The pet in question.

As far as I can find, there are no stat boosters or methods available that decrease intelligence, though there are several ways other than books to increase it, including a handful of books that add more than 1 Intel. You could try asking in the Battle forum in case someone who knows better than me could help

This doesnt seem normal, I would recommend filing a ticket to see if a UA can look into it! There are ways you can gain intelligence without reading a book, so its possible to have way more intel than books read (for example my pet Imperator) but there shouldnt be a way to have less intel than books read.