[ Thanks to An Anonymous Reader for
this link. ]
“There are many good books on GIMP, but you may very well find
everything you need in the way of instruction in Grokking the GIMP.
The book is available in a free HTML tarball in addition to a
printed copy that you can buy. I highly recommend looking at the
links in the contents. This free, online book takes you through
layers, filters, resizing tips, masks, blending colors, case
studies and way more than that. The screenshots and instructions
are from an older version of GIMP, but many of the tutorials will
still give you what you need to perform advanced tasks. (For many
more free books on open source software titles, check this
post.)”