A good post yesterday on Splitsider, about the difficulties of turning your blog (or in this case, your Tumblr) into a book.
I haven't done a ton of blog-to-book deals, partly because the field got very crowded very quickly, and partly because a blog, even a really popular one, is free, and no matter how much people love something, it's not always that easy to get them to shell out money for it. So for me to do a "blog book," it really needs to be something that makes sense as a book independently of the website on which it's ostensibly based. The real acid test: if someone picks up the book in a bookstore (yes, they still exist!), and they've never heard of the website, will the book appeal to them?
Do you spend much time thinking about "target audience" when you write? Do you find that it affects the quality of your work, or its success in the marketplace?