To blog your book without being too boringly obvious or repetitive, consider blogging about the subject matter of your book. People who share an interest in what your book is about could become your readers.
If you write mysteries, blog the latest crime-fighting psychology or tech or techniques. Romance writers can blog on sex, relationships or famous real-life romances. Blog fantasies if you write them, or blog the science behind your fiction if you write Sci-Fi.
So, if you are a writer who blogs, the question could be…
What can you blog about that shares your interests with prospective readers?