Please do not mark or highlight library books! Doing that can interfere with the next person who uses it.
Instead, to help you remember important parts of what you've read, you can use post-it notes. This can do a little bit of damage to the book's pages, but it's not nearly as bad as the alternatives. And of course taking notes on a separate sheet would work; note-taking is known to be helpful in improving comprehension and memory of what you've read or heard.