Stability is most important when choosing a pair of bookends

 One's eyes can marvel at the look of a handsome pair of bookends, but don't forget that a bookend's job one is to provide stability to a line of books.  From the BestReviews article "Best Bookends":

Bookends offer a great deal of support provided they’re designed to be stable. A large, top-heavy bookend won’t offer as much support as a smaller one with a wide base and lower center of gravity. It’s important to make sure the bookend can’t slip. Some come with a slip-resistant bottom for more friction.

 (https://bestreviews.com/best-bookends)