1. You need enough paper to create an object with a noticeable mass that takes time to work all the way through. Too small or short and it doesn't feel worth it. Make it short enough and people could read it in the book store.
2. People are bad at applying a crystalized abstraction in day-to-day life. They are better at learning narratives and fitting the current situation to the closest learned narrative, and then acting out the part of the protagonist. Instead of explaining a statistic or explicit rule of thumb, it would be more effective to give a bunch of examples where someone successfully applies the rule and is rewarded. Those examples can take up many pages.
People felt 5-10 minutes isn't enough time for something as serious as insurance.. but 25-30 is so long it turns people away... And then 5%, maybe even 10% savings isn't enough to go through the effort, but 25% seems unrealistic....
You need a document long enough to seem informative and authoritative without being too extreme in any way... Then you can slap a price on it and call it a book!