The number of people who finish a multi-chapter fan-fiction is, surprisingly, almost always 40-60% of the number who clicked on the first chapter, with the very best reaching 60%, and the misspelled, grammar-free, plot-free, alphabet-soup-vomit of ten-year-olds retaining about 40%.

That means that if you want to write a successful nonfiction book, you don't have to optimize for getting a reader to finish the book but have to optimize for the reader telling other potential readers about the book so that they also start reading.