Gotta assume you're fixing old books to read, not to collect. I completely--but neatly--cover the covers and bindings of paperback books with clear packing tape, trimming it off at the edges with scissors.
Then I open the front and back pages and run clear tape down the crease.
This generally holds them together long enough to get through them once. Oh. It's a trick I learned from the Boy Scouts. They use it on beat up Boy Scout Handbooks.
Wish I could help you on the darned hardcovers. You'll need an old librarian to teach you those techniques.