I have good luck finding ttrpg books on Anna’s archive but that was for Chaosium stuff and Cyberpunk

How many books do they have? On legit sources I only see the one campaign setting book.

For books, I generally recommend MyAnonaMouse. I'm not sure if registration is open currently.

I typically have good luck on Soulseek for TTRPG books. I use Seeker on Android and Nicotine+ on desktop.

I doubt it's the best place to find them but I have found a good few epub/pdf books on Soulseek.

The ponyfinder books are in the amber room on Telegram

Telegram has a 'trove' of excellent TTRPG book groups, you can find pretty much anything you're looking for there.