One of the reference books I use on a weekly basis is the Roark's Handbook for Stress and Strain calculations.
Roark's Handbook
A bit on the pricey side, but it's a lot cheaper than an FEA package. I use it for those "back of the envelope" calculations that you just can't simplify enough. Civil Engineers probably use it on an hourly basis with all the beam calculations in it.
Happy Hunting!
BEN