Agile Modeling by Scott Ambler

Scott Ambler is one of the  best authors of "Agile Development". A good source if the Agile Modeling Site Map, a collection of essays and a general guideline about this methodology. Let's see what Wikipedia says about Agile Methodology:

Agile software development evolved in the mid 1990s as part of the reaction against "heavyweight" methods, like the Rational Unified Process

(RUP). The processes originated from those methods were seen as bureaucratic, slow, demeaning, and contradicted the ways that software engineers actually work.

Initially, agile methods were called "lightweight methods." In 2001, prominent members of the community met at Snowbird (see "The Agile Manifesto," above) and adopted the name "agile methods." Later, some of these people formed the Agile Alliance [1], a non-profit organization that promotes agile development.

Early agile methods--created prior to 2000--include Scrum (in management), Crystal Clear, Extreme Programming, Adaptive Software Development, and DSDM.

Extreme Programming, while it may not have been the first agile method, unarguably established the popularity of agile methods. Extreme Programming was created by Kent Beck in 1996 as a way to rescue the struggling Chrysler Comprehensive Compensation (C3) project. The methodology was refined by Ron Jeffries' full-time coaching and public discussion on Ward Cunningham's Portland Pattern Repository wiki . In 2000, Kent Beck published the first book on Extreme Programming [2]. Elements of Extreme Programming appear to be based on Scrum and Ward Cunningham's Episodes pattern language.