Build a game around each chapter of the book. For instance, after reading the first chapter, the reader has to realize what is the most important word, sentence, concept, etc. in that chapter. He then has to communicate this (website, email, SMS, etc.) to you. If he is correct, he gets a bonus (an extra short story about the main character, a badge, a tip, etc.) If he is not correct or a bit off he’ll get some help to arrive at the right answer. Each chapter has a different game, and if they make it to the last one, they get a big bonus.
The objective is not to make it hard, but fun and rewarding, so they are motivated to continue and maybe also spread the word about it.