Probability in direct marketing

I have put up some calculations at http://spreadsheets.google.com/pub?key=pjtNNMP33Dgum6Yup4umFMw.

It tries to answer a simple question: ‘Would it make sense to mail again?’ using simple probability with some almost arbitrary assumptions. The results are interesting.

Perhaps we’d come the same conclusions by applying simple statistics. Nonetheless, I wonder why direct marketing books don’t include anything on probability. Will it not help in planning? Can anyone suggest some good books or sites on the application of probability, especially basic probability (the equivalent of business statistics), to marketing in general and direct marketing in particular?