Your favorite math reference book(s)?

[Felix Gross]

Had the same experience with Bronstein as Maximilian. Great book, still use it.

Good are also:

The VNR Concise Encyclopedia of Mathematics: The design is straight from the 60ies but its very handy for quickly getting into a new topic or for relearning. More explanation than Bronstein. Lots of numerical examples. Do not know if that is still in print. The original version came from East Germany.

NIST Handbook of Mathematical Functions: Superseded Abramowitz & Stegun.

Oldham et al.: An Atlas of Functions. The first edition had algorithms for programming. Nice graphics.

Emde, Jahnke: Tables of Functions with Formulae and Curves. Precursor to Abramowitz & Stegun. Wonderful graphs drawn without computers.

Felix