Trademarks allow dictionary words. Windows, Apple, Uber... These are dictionary words. Usually trademarks only disallow dictionary words if the business being trademarked is too generically described by that word. For example, a window cleaner called Windows or a produce seller called Apple; that's not allowed.

The point of a trademark is to be distinctive. Unlike copyright, there's no requirement for a trademark to be creative or original. I don't care to find the details of what Facebook is trying to trademark, but it may well be the case that "book" is a valid trademark.