John Edwards' MULTILINGUALISM is an introduction to the phenomenon of 5,000 languages over just a few tens of countries. The necessity to learn multiple languages to function in one's society is, in fact, nearly universal, and Edwards presents the polemic that ensues. The book is written in a very accessible tone and assumes little formal knowledge of linguistics.
Edwards begins by explaining the gradual change of a single language into a whole family of multually unintelligible tongues, the process that gives us multiple languages. He then explains how bilingualism works, how one learns a second language whether as a child or as an adult learner. Code-switching, borrowing, and interference are some important themes here.
The meat of the book is how multiple languages interact not just in the brain of the individual, but among the greater society. Language spread and, closely related, language decline is depressingly reported, while the difficulties of language revival are presented. For Edwards, multilingualism is not just the co-existence of multiple languages within a given society, but even the presence of multiple forms of speech. Very fascinating is his description of prescriptivism, which in seeking to uphold "pure" language asserts that other variants are necessarily impure. And even within a single-language group, men may speak strikingly different than women, as among the Gros Ventre of Montana where men say "jatsa" for bread while women say "kjatsa".
Edwards' MULTILINGUALISM is probably the single best introduction to the topic. However, having been published in 1994, some of its contemporary examples are dated. Bernard Spolsky's LANGUAGE POLICY (Cambridge University Press, 2004) illustrates the matter with some rather more timely issues. Also, Jean Aitchinson's LANGUAGE CHANGE: Progress or Decay is a more profound look the matter than Edwards, and at a similar gentle pace.