Afterword: Learning More
In this book, we've taken the shortest path from "I don't know anything" to "I can solve sort of realistic interacting problems" that I'm aware of. If you read this far, you should know the basic tools for constructing electronic structure programs and have a good basis for exploring more.
The nature of this style, though, is that I skipped vast amounts of material. For example, we assumed ground states aren't degenerate, that the temperature is zero, and that the motion of the nuclei doesn't matter for the solution of our problem. Likewise, we didn't deal with the precise methods used for calculating band structures or many other interesting properties of materials. I also skipped almost all the theoretical aspects of DFT to focus on getting a working code going. I also only treated DFT, though there are various other methods for solving real-world interacting problems.
If you wish to know more, you'll just need to read specialist books. I recommend "ABC of DFT"\ by Kieron Burke for the basics of theory, or "Density Functional Theory: an Advanced Course"\ by Engel and Dreizler if you want a more advanced presentation. For calculating properties of real materials, "Density Functional Theory: A Practical Introduction" by Sholl and Sheckel is a good choice. For the nuts and bolts of materials calculations, "Computational Quantum Mechanics for Materials Engineers: The EMTO Method and Applications" by Vitos is an excellent resource. For wave function methods, there's "Modern Quantum Chemistry" by Szabo and Ostlund.
There are also various other methods, so the interested reader will surely not run out of material.