Great questions! I'm doing this now, actually, on the book-writing side. https://twitter.com/jdawncarlson/status/1304479385647378432
I've done this a couple of times + have lots of thoughts. For me, the blogging was very useful because it allowed me to do a "rough draft" and it gave me some time to figure out if there was a deep enough subject to warrant a book. @jdawncarlson
Over about a year and a half, I did enough blogging on this one topic that I had written about 35k words, which is 1/3 or 1/2 a book, depending.
I gathered together the individual posts in that blog series into a kind of "recap + review" post, so I could see them all in one, that's here, btw: http://www.racismreview.com/blog/2015/12/30/review-trouble-with-white-feminism/
This was useful, too, because it got my work noticed, which led to other opportunities, like writing a column for Huffington Post, along the same lines: https://www.huffpost.com/author/jessie-daniels
So, then I had more words (another 25k or so) and a bigger audience (or, "platform" as they say in publishing). And, that was important to include when I put together a book proposal.
In terms of the actual writing-writing part of it, it was much more akin to the dissertation-to-book process, in which I pretty much started over.
But, --- in both the diss-2-book + blogging-2-book, because I'd just written this other thing on the same topic, the next draft was much better! That's about it. Happy to answer further questions.