You know what? I'm gonna go buy multi-coloured sticky notes tomorrow and set myself to the task of sorting out my library this weekend.
It will require some hard thinking, as some books have been in my possession so long, I don't even remember borrowing them...
Good to hear it! Good luck with the project.
Also, another thing you can do independent of choosing a specific color of the sticky note. Write a circled "B" with a marker for "borrowed" then beneath that, the name of the person. Put it on the front page, sticking up just a little so it's noticeable when looking at the book.