Guest blogging is more easily done if your ebook is non-fiction. You’ll find blogs on every topic known to man. Just google a keyword related to your ebook’s topic and start contacting bloggers, large and small, introducing yourself and offering to provide a free guest blog. In my experience, as you do this you will magically come across a blog or site that has a very comprehensive list of other blogs on the topic, saving you from further google searches for some time.
Guest blogging gets you and your ebook known to other bloggers.
Guest blogging exposes your writing and your point of view to a new audience.
Guest blogging adds link-backs to your site (provided that you are permitted to include a link in the post).
Guest blogging helps establish or confirm your expertise in your field.
A guest blog gives you something extra to promote.
A guest blog may lead to a discussion—more chances to share and be read.
The best thing about providing a guest blog is that it’s never wasted. If you provide one and they decide not to publish, offer it elsewhere or publish it on your own blog.