Sarah G. Thomason's LANGUAGE CONTACT: An Introduction is a textbook for undergraduates. The author, professor of linguistics at the University of Michigan and an expert on certain Native American languages, surveys the phenomenon surrounding two or more languages interacting with each other.
Thomason begins with explaining how language contact happens. Students who don't know that bi- or multilingualism is the norm for most parts of the world will learn of the great bounty of languages in places like India. Thomason describes not only contact in the context of village neighbor multilingualism, but also that between a written religious language and a vernacular, and between the languages of colonial authorities and the local languages of the colony.
Thomason speaks also of "linguistic areas", i.e. Sprachbunds. Her examples include the Balkans, the Baltic, the Ethiopian highlands, South Asia, New Guinea, and the Pacific Northwest of North America. However, my enthusiasm for Sprachbund study has dimmed after reading the papers by Lyle Campbell and Thomas Stolz in the collection LINGUISTIC AREAS in the collection Matras et al. They suggest that no satisfactory definition for a language group can be reached, and it would be better just to concentrate more on the individual borrowings. Plus, Koptjevskaja-Tamm has a paper in there showing that no isogloss spans the entire Baltic, and so a "Baltic linguistic area" is an exaggeration.
For me, Thomason's coverage of the actual mechanisms of contact-induced language change was the most interesting. She presents seven mechanisms: code-switching, code alternation, passive familiarity, "negotion", second-language acquisition strategies, bilingual first-language acquisition and deliberate decision. This is sure to be enlightening even if you have previously read widely in this field.
Two full chapters are dedicated to new languages formed under contact. The first covers pidgins and creoles, benefiting from a great deal of recent research that improve on older thought about the development of languages in the colonial world. The second chapter covers "mixed languages", and is a good introduction to the debate about whether more than a couple of these really exist in the world.
Finally, the last chapter is sadly about the process of language death. Thomason lists ways in which languages dwindle to nothing, such as attrition, grammatical replacement and simple disappearance without much change.
Thomason's textbook is not perfect, but it does cover all the basics in a way that is approachable for readings with only rudimentary training in linguistics.