He didn’t just write books on typesetting, he took a year off of work on Volume 2 of The Art of Computer Programming to _write a new typesetting system_. Naturally, he had to invent/implement vector fonts along the way. But even that wasn't quite enough, so he also took the time to invent literate programming, a style of programming where the source code is also a book with narrative structure to guide the user to a complete understanding of every nuance of the source code. If you compile the TeX source code one way, you get a program for typesetting documents. Compile it another way and you get a TeX document containing TeX: The Book. Same with the Metafont source code as well. All together I think it delayed Volume 2 by a decade.
And then he continued publishing books about typography, in his spare time, for the next few decades.
And then he continued publishing books about typography, in his spare time, for the next few decades.