There really isn't one that I know of. There's the original thesis[0], but it's from a different time and it focuses only on the package management (~ for a distribution) aspect.

Everything else is mostly written to teach people how to use Nix, and the more recent the thing is the more it will focus on surface-level features of the C++ implementation of Nix.

[0]: https://edolstra.github.io/pubs/phd-thesis.pdf