I got a publishing contract with a larger publisher to write a book on Agile (10 years ago, didn't end up publishing, long story).

I know that amazing feeling the author mentions of getting a "yes", while totally ignoring that at best I was going to make pennies. If I recall it was 10% after all publishing costs were paid for. However, I had a full time job, so this was for-the-love-of-it rather than as a real source of income.