Often times I have been asked this question and I answer it the same way every time. Utilize the social networks. Build fan pages that are user friendly; by utilizing the free tools that each site offers you will build your network of readers that enjoy the genre that you write within.
Join forums and talk about your book.
The bottom line is utilize your online appearance: Remember time is money so the more time you spend the higher chances you will have in making your self known as well as your work by doing this you will make your book visible for others to see and purchase