GIMP an image manipulation and paint program for Linux / UNIX. The GNU Image Manipulation Program (GIMP), is a raster graphics editor application with some support for vector graphics.
GIMP can also be used as a paint program. It features a set of drawing and painting tools such as airbrush, clone, pencil, and paint brush. Painting and drawing tools can be applied to an image with a variety of paint modes. It also offers an extensive array of selection tools like rectangle, ellipse, fuzzy select, Bezier select, intelligent scissors, and select by color.
GIMP offers a variety of plugins that perform a variety of image manipulations. Examples include bumpmap, edge detect, gaussian blur, and many others. In addition, GIMP has several scripting extension which allow for advanced non-interactive processing and creation of images.
This is a free book (tar ball version). The author illustrates the use of the Gimp with clear diagrams, easy to follow descriptions, and excellent set of examples of the Gimp's functionality are provided. This books goes very well with official Gimp documentation :)
You can also purchase this book for reading offline.