We're going to see the Dark Knight today. Everyone says it's a great movie, so I'm really looking forward to it.
Word of mouth is a powerful tool not only in the movie industry, but also with books. A publisher's backing, as well as a well-thought out author's marketing plan, can contribute to book sales, but ultimately it's recommendations that seem to sell the most books.
If people have read your book and like it, thank them for the compliment. Then mention if they really enjoyed it, would they please tell a few friends either in person or online.
Also, it doesn't hurt to donate your book, especially to booksellers. If they like it, you've got a great selling campaign going.