I'm using Pixelmator instead of Photoshop. The problem is there are no books on learning Pixelmator. Is there a book that teaches design techniques that is not centered around Photoshop? Where can I read about "layers" "masking" and the like without needing Photoshop?
In my opinion no quality resource covering "Design Techniques" will center on ANY specific software.
Software is merely a tool. You wouldn't expect to learn how to be a plumber by reading how to use a wrench.
If you are looking for specific image processing steps you could start with the Pixelmator web site. After that, you can often simply Google "Pixelmator tutorials" and find several hundred pages.
As with any software, the tutorials/books/guides are really only to teach you the tool. Once you learn and are comfortable with the tool, the design is up to you. The goal should be to move past tutorials and create things yourself rather than following tutorials.