This is a comprehensive inventory of virtually every language known to linguists, using genetic classification to classify them by their linguistic family and tracing their roots all the way back to proto-languages, where possible. I don't know of any other book with a similar scope and comprehensiveness. Language classification is fuzzy sometimes, so not all linguists will agree with Ruhlen's divisions.
Obviously, this is a reference book, not the sort of thing you read cover to cover. The charts, tables, indexes, and maps are what most people will be interested in. However, for linguistics researchers, there is some discussion of methodology and the field of historical linguistics.
There was a promised volume 2 and volume 3, but as far as I know, these have never been published.