About getting out of one's place, I remember seeing a lot of kids sitting on the floor reading comics in bookstores. I was one of them. It was well tolerated as long as you weren't in the way (if you were, they just politely asked you to sit somewhere else). I don't know if it still happens, it certainly doesn't look as common but maybe that's just me not paying attention.

Of course, by spending way too long in bookstores, I often ended up actually buying books. I used it essentially like a library, except I bought books to bring them back home instead of borrowing them.