Leaving aside the question of the nature of its jokes, the TeXBook is the sort of thing you read straight through. It's a narrative about how some code came to be. It's not the same sort of reference as the glibc manual, and I think a narrative form of documentation benefits from a casual tone and a four-sentence standalone reference about a function (especially where one of those sentences is "Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe lock corrupt") benefits from a terse tone.
Knuth has a great sense of humor in his books and exercices. I also like Stallman's jokes, but I can concede that they are sligthly less funny than Knuth's.
