He encouraged people to share the book and it's ideas with everyone they could. It quickly became a hit and people started talking about it and more and more people downloaded it. A good majority of the book's readers recommended it to someone - who recommended it to someone else and so on making him a pretty popular and well known guy. He had little to because his readers did the hard part for him. And people were happy to do it because the product was of excellent quality and people felt comfortable in suggesting it to others.
Word of mouth equals viral marketing.
They key to viral marketing is to give them something to talk about and share. Once you come up with a great idea or product - the hard part is done. I recently purchased a Flash music CD from Mark's site and am just thrilled with it. It's very inexpensive and has 50 top quality - royalty free music loops. I want something special to use.