Not only older issues of the Numerical Recipes books, but also classics such as
Handbook Abramowitz and Stegun, Handbook of Mathematical Functions (10th corrected printing, 1972)

Bateman, Erdelyi et al. (Bateman Manuscript Project)
Higher Transcendental Functions (vols. 1, 2, and 3)

and
Encyclopaedia Britannica the great 11th Edition (1911)

Found via https://www.hpmuseum.org/forum/thread-16251-post-142741.html#pid142741