Library Books connects to your library catalogue and downloads the list of books you have checked out. Then books are then displayed on the menu bar. You can then view the books you are currently borrowing in the menu bar as in the picture below from my computer.
If you look carefully the green star next to the number 8 indicates 8 books on loan, the green star indicating I have one book on hold ready to pick up from the library.
If you have a Mac and regularly borrow books then this is a great application and the price is right.