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.