In the early 1980s, I inadvertently agreed to produce a 100-page manual for the student programming environment I had designed and implemented at the University

In the early 1980s, I inadvertently agreed to produce a 100-page manual for the student programming environment I had designed and implemented at the University of BC. I stupidly decided to do it in TeX. The stupidity here was that (a) the only laser printer on campus was in another building and (b) the TeXBook didn't exist yet. All I had was a listing of a late prerelease of the Web source of TeX, along with some of Knuth's writings about TeX77, which was a substantially different system. I managed to do it, including formatting much of the manual in the style of Unix man pages (the environment itself lasted for several years, until mainframe timesharing for students went away for good). When I finally got my hands on the TeXBook, I found that I had learned much about TeX, including a few things that weren't so :). Knuth's programming style is highly idiosyncratic, and there are many points with which I'd disagree. Furthermore, the choice of Pascal required all sorts of strange compromises. That said, I highly commend reading the source of TeX, to see how a brilliant computer scientist attacked a problem that is not nicely structured, using very restricted programming tools. You will not get much that you can copy into your programs, but it is an excellent, well-documented attack on an interesting and complex problem. (By the way, Knuth's TeX was written long before modern typesetting things like Unicode, PDF, and OTF fonts; you don't want to use Knuth's TeX for modern work; I use LuaTeX. But Knuth's version remains as a useful subject of study.) Another good thing to study is Lions's commentary on Unix V6. This is interesting because it shows many of Unix's key abstractions implemented in a few thousand lines of code.