Not a book (you don't really need a whole book for such a small language!), but this is the classic resource: https://nixcloud.io/tour/

Then the things you need to learn in order to read and understand most Nix code is just the stdlib, which is most convenient to browse here: https://noogle.dev/

Beyond that, it's all just conventions that are covered well by reference material. So those two are all you need in order to find your footing when it comes to the Nix language.

(For a tutorial overview of the whole ecosystem, nix.dev is the best entrypoint. For the build system or an introduction to some of the basic "internals", you want The Nix Pills. And in general, the best thing of all to read when it comes to Nix is Nixpkgs itself. :)

nix.dev is generally pretty solid. Check out their tutorial for the Nix lang: https://nix.dev/tutorials/nix-language