Yup, I started blogging specifically to promote my book. I hardly ever post about writing, though -- I mostly post about things that I think people who would like to read my book would also be interested in, on the theory that if they like reading my blog, they'll like reading my book!
About guest posting -- I've invited three different people to guest post on my blog, none of whom have taken me up on it yet, but all of whom have said they want to at some point when they think they'll have something to say that would fit well on my blog. I'm SUPER picky about whom I would ask to guest post, because my blog is my brand and my thoughts and I'm implicitly endorsing anything posted on it. All of the people I've asked are people I know and am secure would write thoughtful posts that would add to my blog in wonderful and positive ways, and there won't be any awkward conversation about editing their posts.
*shrug* Some people are less picky. I think it depends on how you feel about your blog space and how closely it connects to your brand. Jim Hines said something similar once about giving people quotes for their book covers -- he wanted to be careful about not endorsing anything too far from his own brand (i.e., too racy/violent), even if he liked it, because he was afraid his (younger) fans would see his endorsement and assume the book was appropriate for YA readers.
As for guest blogging other places -- I've had articles crossposted a few places, usually from my own submissions (emailing a site that invites submissions with an article relevant to their interests). This is easy, because since I have interests in common with those sites, I was already reading them and also already writing about the same topics. I just had to look up their submission guidelines. I also got made a contributor on another blog by doing this. I find that as long as you follow good etiquette and are sharing a genuine interest, people respond well and it's a lot of fun (in other words, only inquire about guest blogging or crossposting at sites that (a) invite such submissions and (b) are *specifically* tied to your interests; I think it would be obvious to spam a bunch of unrelated blogs and ask to guest post).
Anyway, just my 2 cents so far. I'm a pretty new blogger (since August), so take that into account.
Good luck!