There are chapters on most of the important languages of the world, as well as some of the language families that include these languages. Each chapter is by a different expert (actually, a few people wrote more than one chapter), and so there is some unevenness in the treatment. But in general, each of the single-language chapters gives a relatively detailed summary of the grammar and vocabulary of the language it covers; the language-family chapters describe the common features of languages in the family. The level of detail is not that of a textbook in the language, but rather enough to give someone like myself (interested in linguistics, but not fluent in anything but my own native English) a good feeling for how the language works.
The first book of this type that I ever saw was Mario Pei's "The World's Chief Languages." This book goes into more detail on any individual language than Pei's book did, but covers a smaller number of languages (though more varied ones). It belongs in the library of anyone who wants to know a little bit of how a lot of languages work.