Agile testing is a software testing practice that follows the principles of agile software development.
Agile testing involves all members of a cross-functional agile team, with special expertise contributed by testers, to ensure delivering the business value desired by the customer at frequent intervals, working at a sustainable pace.
Classic books include:
Lisa Crispin and Janet Gregory's "Agile Testing" - the site has two chapters available to download.
Elizabeth Hendrickson's "Explore it!".
See also her "Agile Testing Overview" (pdf) for a quick and very readable summary.