Starting a blog to promote your book is a good idea. Sometimes the information you blog about may be great for your actual book. Building conversations and relationships through your blog is an excellent way to create new content that may be more dynamic than you simply writing what you think your readers will want to know. Blogging could be sort of real time market research. Isn't that something?
Consider when you converse with people on your blog you're finding out exactly what your target audience wants and/or needs. This allows you to write and publish a book that's truly meant for the market place as opposed to what you think people desire.