I'm just getting started with an interesting book I found at the library the other day. I've only scanned through it. The book starts out very basic but seems to get into some very good explanations of more advanced topics.

Beginning Digital Image Processing
Sebastian Montabone
Apress
ISBN-13 (pbk): 978-1-4302-2841-7
ISBN-13 (electronic): 978-1-4302-2842-4

It features, Gimp, UFRaw, Qtpfsgui (Luminance HDR), Hugin, Imagemagick, Exiftool, Resyntheziser, Avidemux.