These are books that my uni courses never had me read. I'm a little shocked at times at how my degree program skimped on some of the more famous texts.

It is not a textbook, it is an extremely dense reference manual, so that honestly makes sense.

In physics grad school, professors would occasionally allude to it, and textbooks would cite it ... pretty often. So it's a thing anyone with postgraduate physics education should know exists, but you wouldn't ever be assigned it.