GIMP Book - Beginning GIMP - From Novice to Professional, Second Edition by Akkana Peck
GIMP is a great image manipulation program. Best of all it's FREE 
And there is a good community of people writing little scripts and filters for it.
It is the free open source competition for things like Photoshop.
If you are not 'expert' at using it, and want to learn more, this book has been recommended to me:
Beginning GIMP: From Novice to Professional, Second Edition (Paperback)
by Akkana Peck
It certainly gets very good reviews.
From first installation to advanced image manipulation techniques, Beginning GIMP: From Novice to Professional, Second Edition explains how anyone can utilize The GIMP: a free, open source image manipulation program. You will learn how to install The GIMP on Windows, Linux, and MacOS X platforms. Once you’ve installed the application, you’ll learn about the interface and configuration options, and then jump into a quick and simple project to familiarize yourself.
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With four–color graphics and screenshots throughout, you’ll learn how to prepare camera images for display on web pages—including processes like rescaling, cropping, and balancing color. Utilizing layers, paths, and masks are also covered in extensive detail. Of course, you will also learn how to draw lines and shapes; utilize patterns and gradients; and even create your own brushes, patterns, and gradients.
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GIMP expert and author Akkana Peck teaches you how to touch up digital photographs, smudging away blemishes, fixing red–eye, and stitching panoramic images. You’ll learn how to tap into the powerful filters, effects, and plug–ins that are available for The GIMP and automate tasks using scripts. The entire book is approached in a project–based manner, and as you progress, numerous projects will help strengthen your newly acquired abilities.
What you’ll learn
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* Install The GIMP on Windows, Linux, and Mac OSX.
* Explore the interface and configuration options.
* Prepare your camera images for use on the Web, including rescaling, cropping, and balancing color.
* Learn basic techniques such as drawing lines and shapes, utilizing patterns, and making use of gradients.
* Master advanced techniques such as layers, paths, and masks.
* Create your own brushes, patterns, and gradients.
* Discover tricks for fixing blemishes, removing redeye, and stitching together panoramic images.
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Who is this book for?
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This book is for graphics designers, digital photographers, and hobbyists. It’s is aimed at those who need to utilize a full–featured image manipulation program but don’t have hundreds of dollars to pay for Photoshop. The GIMP is also the preferred image manipulation application for the open source advocate. The GIMP is available on Windows, Linux, and Mac OSX.
And here is a sample video:
Akkana Peck Layer Mask Tutorial using GIMP
http://www.youtube.com/watch?v=pEGR_Qps9OQ
Good explanation 
HT Apples
This is the main GIMP website:
http://gimp.org/
For Windows, you can get it from here:
http://www.gimp.org/windows/
When I last got it, you needed to download a set of files:
The Gimp for Windows (ver 2.2.10) (7.863MB)
GTK+ 2 for Windows (ver 2.8.9) (3.734MB)
Help 2 (ver 0.9) – optional (21.994MB)
Gimp Animation Package (ver 2.2.0) – optional (2.542MB)
That's a while ago now!
You can learn some fantastic tips from tutorials that users have created.
http://www.gimp.org/tutorials/
http://www.gimptalk.com/forum/gimp-tutorials-and-tips-f8.html

This is the book I have.
Grokking the GIMP by Carey Bunks
NZ$93.42
UK: US:
The reason I got this book is that I could read it online first to see if I thought it was good.
It's always nice to have the book in your hand.
Check it out here:
http://gimp-savvy.com/BOOK/index.html
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