I originally built this as a b-day present for my wife, and she liked it so much that I decided to turn it into a public service. And I'll be happy if it's just that.

But we do have a model to generate (modest) revenue, which hopefully will cover the (again, modest) costs: At the end, when the recipient has seen all the letters, s/he can order an actual, physical book with all the text. It's printed on-demand, and we're make a bit of $$$ over what the publisher charges.