Is a book publicly available? No, you have to purchase it. But once you do, you're legally allowed to let your friends and family and so forth read it too. As long as you don't sell copies of it (the "copy" part of "copyright"), or meaningfully take away the ability for the publisher to make money from sales (so you can't post it for the whole world to see on the internet).

And sure, there are lots of ToS for digital works, but are they actually enforceable? ToS can say you're not allowed to let anyone else read the book you purchased. But no court is going to say you can't lend your Kindle to your friend for them to read it too. Many ToS clauses are flat-out illegal.

Meta will argue that training on books is no different from reading all the books at a friend's house. That as long as Meta isn't reselling or making publicly available the original text, they're in the clear.