Convolution (including via-FFT where it's mainly just complex-multiplication). Different neighborhoods and operations on them: Laplacian, curl/divergence, derivatives/slopes, completely-custom-weights/operations etc. Non-raster (ie analytical) convolution is something I find fascinating and haven't experimented with.
Noises both in the Perlin sense and the Blue-noise sense. Such a fundamentally useful building block!
Dealing with anisotropy both in shading, and texture filtering.
Color spaces. Hardly need to say more than that (apart from maybe 'HSV' should get a solid go). I think getting some solid clarity on what 'linear-to-light- color spaces are and how and when to work with them, is a critically important thing that is very often overlooked.
Acceleration structures - you often see stuff written about these for 3d (to speed up ray-intersection-tests for example) but I think there's probably lots of interesting acceleration stuff for 2d graphics too!
Feedback (ie 'solvers') in the 2D pixel realm - Game of Life (I have HashLife in mind) is just the tiniest-tip of the iceberg there, and Reaction Diffusion takes some solid steps but there's also repeated-warping (ie cheap 'advection') and repeated neighborhood-based pixel/cell operations that I think are an under-explored area.
Complex numbers and Orbits-based fractal-rendering. Mainly because I think it's fascinating, but also very useful for getting more comfortable with complex numbers and antialiasing these beasts that can indeed cause a LOT of aliasing!
Lots-of-points style fractal/etc rendering (eg Flame fractals, IFS, Buddhabrot, that kind of thing, or just HEAPS of particles!) Including how to cope with zooming-in when using that rendering scheme (I have not used it myself but I heard-about and saw great things that used 'Metropolis sampling' in order to make zooming practical.)
I think it would be great to include a section covering all the amazing work that originates in the demoscene and all the great techniques that have come from it. ShaderToy is one example of this, and there are many more.
That's enough for now! Best of luck with your book! Happy to talk further if u want! Email is on my profile.