I took up writing as a personal challenge, so the "test" angle really resonates with me, as does the altruism angle--the writing is worth more to me if it can entertain or help others.

The eBook--especially in a shorter form--seems like a natural thing to write because we do plenty of purpose-driven research as writers. Just adding organization and polish could make the research into a marketable product. There's also the opportunity to provide a resource that you think is missing from the market.

I'm currently putting together two eBooks and teasers for each, so this article is quite timely in reinforcing my desire to do so!