> The language is impossible to learn, and my limited experience has been that there are extremely few good guides to peering under the covers.

Maybe my "Nix from the bottom up" page would help? It peers so far "under the covers" that I only introduce the Nix language very briefly near the end (as a saner alternative to everything else on that page!) http://www.chriswarbo.net/projects/nixos/bottom_up.html