The question of where to find reliable information on language classification came up over at Language Hat recently in the comments on this post about some beautiful new language maps that, regrettably, are based on unreliable linguistic classifications. The problem is that some commonly used sources, such as Charles Frederick Voegelin and Florence Marie Voegelin's Classification and Index of the World's Languages (1977), are badly dated, while others, such as Merritt Ruhlen's A Guide to the World's Languages (second edition 1991), are unreliable for other reasons. Ruhlen's book is generally fairly accurate for the lower levels of classification but is quite unreliable at higher-levels due to its reliance on an unsound and subjective approach to linguistic classification.
The best single source of information is the Ethnologue, a publication that lists all of the known languages of the world by region. For each language it provides the classification along with such information as where it is spoken, by how many people, and its endangerment status. The Ethnologue also provides numerous maps showing where languages are spoken, and contains an extensive index of alternative names, since many languages are known by several names. (Because the sponsor of the Ethnologue, the Summer Institute of Linguistics, is an organization whose primary purpose is the translation of the New Testament, entries also indicate the availability of the Bible in the language.)
The classification in the Ethnologue generally reflects the mainstream view of historical linguists. It is occasionally somewhat out of date, and is sometimes criticized for treating what most specialists consider to be dialects as distinct languages, but overall it does a better job than any other reasonably comprehensive publication.
The printed version is accompanied by a CD-ROM and has colored maps. It is also available on-line. There are several indices:
The best way to go beyond the Ethnologue, either to obtain what may be more up-to-date information or to find out more about the reasons for decisions about classification and the problems and controversies in particular cases, is to consult specialized publications on particular areas of the world and language families. Here are some good sources: