You should purchase a domain name for your blog whether or not you plan on having a hosted blog. Should you convert from a free blog to a hosted one, you’ll need it. And if you begin with a hosted blog, obviously, you’ll need it.
Purchase the same domain name as the title of your book. (While you’re at it, purchase a domain name that reflects your own name as an author. For example, I own ninaamir.com as well as howtoblogabook.com.) If you can’t get a .com name (ex. howtoblogabook.com), get a .net name (ex. Howtoblogabook.net). If you can’t get that, pick one of the others. I typically think of .org as something for nonprofits and such, but nothing stops you from using that. Some people purchase every domain name possible so no one else will get it.
You can purchase domain names in any number of places. The cost may vary. Check them all out. Many places, like GoDaddy.com, offer cheap rates and hosting as well. You can do a Google search and then comparison shop. Or, better yet, ask someone savvy about websites for advice.