NIST Digital Library of Mathematical Functions
dlmf.nist.gov
dlmf.nist.gov
The scan of the classic version can be downloaded as a PDF from http://www.cs.bham.ac.uk/~aps/research/projects/as/book.php.
Then, during some of my courses on numerical analysis, I like to brandish the book in front of the students, and explain to them how awesome it is.
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).