Voted one of the "100 or so Books that shaped a Century of Science" by American Scientist it is a multi-volume work of astounding comprehensiveness regarding the most fundamental aspects of modern programming.
It is not a book on patterns, architectures, or even software engineering. It's a very low-level look at the very nuts and bolts which make any program work from sorting algorithms to random numbers to trees and lists. It sounds almost trivial but the level to which the topics are analysed and described is wonderful in the true sense of the word.
Not a book (series of books actually) to simply read or to spend 5 minutes looking through on occasion. It's a hard slog to read but it covers topics in such a complete and definitive way it remains a valued member of my personal library. The level of difficulty ranges from "warm-up" exercises to unsolved research problems.
Bill Gates is quoted as having said: "If you think you're a really good programmer... read (Knuth's) Art of Computer Programming... You should definitely send me a résumé if you can read the whole thing."
And while Knuth was frustrated at dealing with the publishers of Volume 2 who didn't like doing all the Math typesetting, he said "Well, why don't I create a Document-creation system that can "do math" and have flexible fonts? I'll call the Doc-creation TeX, and pair it with Meta-Font". My kids asked me about 6 years ago if I'd ever heard of LaTeX, and I said "You mean the system that Donald Knuth invented about forty years ago?" Silly me, I didn't realize it was now available (for free!) on the PC. Goodby, Microsoft Word ...