I agree with PSlem - Roark's book (its my first choice for this type of problem).
For a simple solution - flat plates subject to a uniform pressure - have a look at 'Design of Welded Structures' by Blodgett (the first of the Lincoln Arc Welding books). Formulas for circular, square & elliptical plates are in the section on Tanks, Bins & Hoppers - Table 1 (p6.5-3 in my copy).