I run Correlated (http://www.correlated.org), a site that publishes one surprising correlation a day, using data generated by readers.
It was never really intended to be a money-making project, although I did give display ads and affiliate links a try, with very little success.
And then ... a book deal fell into my lap.
I had been shopping around a book proposal for "Experiments on Babies" (http://www.experimentsonbabies.com), and one of the publishers that was interested in that book also happened to note that I was the creator of Correlated, and asked if I would be interested in a separate deal to turn Correlated into a book.
I got a very nice advance for the two book deals, and in the case of Correlated, the writing involved is, for the most part, what I'd be doing anyway, deal or no deal.