About firming agents:
http://nchfp.uga.edu/how/can_06/prep_foods.html

You can also use alum, which is typically found near the spices/salt, etc. in ordinary supermarkets. I was wary of using it, but last year made some bread and butter pickles, using the recipe on MrsWheelbarrow's site, and they turned out great -- nice and crisp! Love that crunch of crisp pickles on a sandwich!! ;o)

One traditional method is to use pickling lime (calcium hydroxide) but the process is laborious and dangerous if you don't get it right.

Ball, the canning jar people, makes a product called Pickle Crisp (calcium chloride) which is safe and easy to use.