NIST Handbook of Mathematical Functions , by F.W.J. Olver et al (eds.)

This is the latter-day version of the original Handbook of Mathematical Functions by M. Abramowitz and I. Stegun, which is still available as a Dover edition. It is an invaluable reference on special functions. An online version that's in the public domain can be found here , and a scan of the original A&S version is here .