The U.S. National Institute of Standards and Technology (NIST) have a killer resource for all math functions organized by discipline.
NIST Digital Library of Mathematical Functions
NIST and Sematech published an comprehensive free e-book on statistical modeling analysis and how to apply it to engineering and scientific work.
The Case Studies chapters in each of the sections are of special usefulness.
NIST/SEMATECH e-Handbook of Statistical Methods