I used to go to the library, find books with the relevant chapters related to what I wanted to learn, and the librarian would photo copy all the pages I wanted to take home. So I guess technically you could copy all the books for your own use.
It's just impractical to photocopy every page of every book in a library.
When I was in libraries you could photocopy a percentage of a book (15% maybe?), although I doubt it was enforced. One could do many trips, but it is impractical, as you say.
It's just impractical to photocopy every page of every book in a library.