If you're an author, you've likely heard over and over, "you need a blog." Do you really need a blog? The ultimate question is, will having a blog help sell books? The answer is, it depends (there's never an easy answer!).
Blogs are all the rage, and everybody seems to have one. Your sister has one, your best friend has one, and even your child's school has one. But the important question is, who is reading them? A blog is only useful if it succeeds in attracting a significant number of visitors who faithfully read the various posts. Hopefully, a number of those visitors will turn into buyers of your book.
So, the next question is, how do you attract visitors and how do you keep them reading your blog? First, just like a website, you need to advertise your blog by getting other sites/blogs to link to it. Use the address in your name signature for your email lists, etc. Basically, do what you do to advertise your website. Next, you MUST have something to offer your readers. Commenting day to day on insignificant (to them) events related to your book (hey, I just got a review!) will lose readers quicker than you can say "read my blog!" Offer articles related to your book's topic (after all, that's the topic that readers have come to your blog to learn about). Comment on current events related to your topic/genre. Perhaps an occasional post linking to other sites that have something interesting to say on the topic. Finally, you must post frequently. The whole point of a blog is to offer up-to-the-minute information. Posting once or twice a month is useless.
If you honestly don't have a lot to offer on your book's topic (and it is harder than it might seem) to continually post to a blog, then you might be better off spending your limited time on other apsects of marketing. Having a half-hearted blog that few readers subscribe to will not help your image, nor will it sell books.