Blogging is like being the hostess or guest of honor at a party (or maybe just my kind of a party…): everyone is already connected to you somehow; you have some measure of established status, credibility and sometimes feedback.
Whatever you say (write) has the general context and cushion of relationship to get you to the next post/conversation, so flubs are less-threatening.
Writing a book, on the other hand, is like giving a speech. In the same way that collecting three points (along with their supporting material) takes for most minds more thought than daily conversations, you think longer about the words for a book.
These words have more weight to you as a writer because they must be solid enough to stand on their own. This work will be read by disinterested (in you the writer) people as well as those who love you and those who’d delight in ripping a gaping hole in your idea-baby.
I was thinking about a similar thing myself the other day…I think this is the same distinction I feel about speaking about a controversial subject to a crowd where I am the invited speaker vs. to a small group where I am piping up about my (usually unpopular) opinion. Though I am an introvert, speaking to a large gathering wears less on my nerves when I am set up as the respected expert (even if not everyone there agrees with me, as is often the case, the “organization” hosting them agrees with me, so they have to listen) than when I’m talking to even one or two people who are hostile to the ideas I’m presenting. Though I have the same confidence in my position and research, I worry much more about the words in the second instance.