
It’s the digital successor to the Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables by NIST’s predecessor NBS. It’s a classic reference which google scholar says has like 120k citations. It’s colloquially called “Abramowitz and Stegun” after the editors. My pet hypothesis is that it’s so well known partially because “Ab...” tends to put as the very first entry in most of those 120k reference lists.
It’s a super handy reference for all sorts of various properties of special functions. To say it’s extensive is an understatement.
EDIT: oops, just realized that you were referring to a later version of the classic. I was referring to Abramowitz and Stegun's Handbook of Mathematical Functions with formulas, graphs, and mathematical tables (1964).