I wrote recently from Bosnia and Herzegovina about the curious practice of taking a unitary language and trying to find ways of representing it as several different languages for political reasons, in order that each of several ethnic groups should be able to claim a tongue of its own. I wrote on the basis of my own experience in the country rather than delving into reference books about it. But after my return I checked the classic reference work on the languages of the world: the Ethnologue.
The Ethnologue is published as a book by the Summer Institute of Linguistics, in numerous editions. The latest edition is the 16th, published in 2009 (see the Amazon.com entry for Ethnologue: Languages of the World). It incorporates the ISO 639-3 standard inventory of three-letter language identifiers (you are currently reading ENG, of course).
But the Ethnologue now also exists as a Web site…