Open Source animation software : Pencil, Synfig Studio

http://www.pencil-animation.org/

Also see:
http://www.synfig.org/cms/
Synfig Studio is a free and open-source 2D animation software, designed as powerful industrial-strength solution for creating film-quality animation using a vector and bitmap artwork. It eliminates the need to create animation frame-by frame, allowing you to produce 2D animation of a higher quality with fewer people and resources. Synfig Studio is available for Windows, Linux and MacOS X.

Books + tutorials:
http://www.desktopacademy.com/store/