One popular method of marketing books online is through a blog tour. Think of this as a virtual speaking tour. Instead of travelling to different cities to speak to audiences about your book, you simply write a guest post for the audience of the blogger hosting you. Variations on the blog tour include author interviews and giveaways. The whole idea is to let a new audience know about your book.
I just read this post on setting up a blog tour. Wish I’d know some of this when I was setting mine up!