Large, expensive American reference book on languages.

Well printed and nicely bound, it covers over 400 languages. Is that too many? Given the space, each language gets only one or two pages. The description is certainy well researched and serious, but if you want to learn things about a speficic language that you are considering to learn, you will probably find it too short.

If you want a universal catalogue of human languages, to briefly learn quick facts about unknown languages, this is an authoritative resource, although you will probably find more material on the Internet. But if you want a book to help you decide which language to learn, how difficult it is, how you can use it, and get extensive insight about the language peculiarities from teh learner's perspective, this is not it. If you want a catalogue of languages you can learn and you can read French I'd recommend you get Malherbes instead.