Thought this idea was interesting.
Jason Fried, founder of 37signals wrote a book called REWORK. He talks in it about the fact that when you make something, you always make another thing. There is always a by-product. Sometimes you can also sell the by-product. His book itself is an example of a by-product from creating something else - his company. And I suspect the book will create another by-product in the form of seminars, workshops and consulting perhaps.
Dave Trott did something similar. His (wonderful) book is a by-product of blog posts. His blog posts are a by-product of all the teaching and advertising he has created over the years.