From what I understand, TAoCP is building up to that compilers book! When initially tasked with writing about compilers, he realized all the background necessary for building them and planned out these books as an overview of computational techniques that would be helpful to use.

See the "future plans" section of his website: https://www-cs-faculty.stanford.edu/~knuth/taocp.html has volumes 6 and 7 as being on compilers.

danke, freund