A little while ago, I wrote about just how many languages there are to learn and John McWhorter’s Language Families of the World. Today, though, I stumbled upon something truly incredible: Handbook of Descriptive Language Knowledge. The author, Harald Hammarström, was good enough to upload this fantastic guide to Academia.edu. In it, he provides categorizations, by region and language family, of pretty much every language for which documentation exists, along with references to the newest or most comprehensive studies of that language. Here’s Mongolian, for example:
3.19 Mongolian (14)
Core area: Mongolia
Canonical source: 
 Janhunen, J. (2003b). Proto-mongolic. In Janhunen, J., editor,
The Mongolic Languages, Routledge Family Series, pages 1-27.
Routledge, London & New York
Janhunen has written an excellent handbook with many sketches . A
recent grammar is .
 Janhunen, J., editor (2003a). The Mongolic Languages. Routledge
Family Series. Routledge, London & New York
 Slater, K. W. (2003). A grammar of Mangghuer: a Mongolic language
of China’s Quinghai-Gansu Sprachbund. Routledge, London & New York
If you’re studying a language like French or German, of course, there are lots of non-academic resources. But if you’re interested in a language that’s way off the beaten path, this is a great place to start.