I always use the usps website to calculate, Art, the one Sharon listed. I know there are a bunch of dumb questions, but I can usually suss out what would be usually more than enough by using that.

over at Paperback Swap, they've got a nice system which calculates it for you AND, if you want to pay them, they will even let you print out the wrapper and postage for any book you might be sending out. It's pretty neat.

Do you have a mail carrier who might be of assistance?