Is it better to gain intelligence now from reading books or to slowly improve intelligence from intelligence experience points (from playing mystic spindles)? Since we can only gain max 400 intelligence points from playing daily, it would take a few days to raise intelligence by 1 point, but there is a limit on reading books (since you can only read one book once per pet). Would appreciate any advice from experienced battlers

If you like playing mystic spindles, do that first. The higher your pet's intelligence the more points it takes in mystic spindles to raise it further. When it starts feeling unproductive to play the game switch to reading books. If you don't feel like playing the game you can skip it entirely and just read books and use stat boosters once books get expensive.

SubetaLodge has a great feature to tracks what books your pet has read and can sort by location as well as lowest price in user shops.

Thank you! That's helpful!