Forget about selling the book itself, use the book as a way to find new fans. If half those 800 people each week signed up for her email list or followed her on Twitter, she would have a huge fan base which should could tap to FUND HER NEXT BOOK on Kickstarter.
Sell the making of the content, not the consumption of the content.
The hardcore fans will want her to write the next book, so they will fund it. Some percentage of the random people who download her book will turn into hardcore fans. It's a scalable, repeatable cycle.
400 people a week = 20k in a year. 5% of that giving $15 is $15,000, her entire advance.
EDIT: I did basically this model for documentaries and built an audience of a million members.