Besides marketing campaigns and using promotional services, there are some secret ways to promote your book. Caution: use in moderation!
1) We all have Twitter right? Well if you are an author you should have! Plenty of readers on there. But also there is something known as FollowFriday (#FF). Where a post on Friday can have many people tagged in the conversation and it is socially acceptable. You can choose a link to your blog, an interview, book promo, and tag potentially interested people. TIP: Don't always have links to your books because word will spread and soon people who you tag won't even bother favouriting or retweeting. They won't even click on the link. When it comes to content on Twitter, share a wide range of material. From quotes, news, inspiration, images, personal details, hobbies and other author's books. Don't always self-promote, it gets old fast!
2) There is a similar thing that can be done with Google+. When you share a blog post you can tag as many communities or people as you please (but again, don't overdo it).
3) Also I have found that subtly discussing your book on your personal social networking accounts (Facebook, Twitter etc) can be rather effective. But you have to say something about the themes, something comical or that you are happy the first draft is almost complete. Or when it's published, say something personal, entertaining or humorous about the book.
4) Locating where your readers are mostly likely to hang out online and socialize with them. Don't blatantly self-promote, build a relationship so they trust that you write quality material. Even suggest your blog where they can read samples from your work or free short stories (ONLY IF THE OPPORTUNITY ARISES!)
5) Use free short stories. Promote short stories with the intended purpose of giving readers in your genre a free read. But place ads on the blog or site where the story is available, as to advertise your work. And if they enjoy the story, chances are they will buy at least one of your books for the right price.
6) This is one you've no doubt heard a lot, but have more than one book available! If you haven't published any books yet I would wait until you've written at least three. Then when they buy and finish one they can buy another from you. In the meantime you can be churning out a fourth and fifth. If not they will move onto another author and probably forget you even exist.