The author's enemy is anonymity.
Without marketing, your new book has a good chance of remaining forever anonymous and unread.
Steve Weber's Plug Your Book! Online Book Marketing for Authors, Book Publicity through Social Networking is a well-written explanation of how you might market your book.
What makes Weber's book vauable is his up-to-date knowledge of online promotion. He uses the techniques that he describes in Plug Your Book! to sell his own books.
The Amazon information alone is worth the price of the book. Highly recommended.