Blogging is an essential part of book marketing. It’s always a bit mystifying to hear authors say, “I don’t know what to blog.” You’re a writer! It’s part of the job. If readers like the way you put together a post, they’ll be more likely to check out your book.
However, blogging can also feel like writing into the wilderness, especially if your site gets scant traffic, which is usually the case for new writer’s sites. If your blog (or book) is hurting for viewers, a good practice is to post on a blog with a built-in audience.
CreativIndie has this to say about guest posting.