It seems likely to me that if user pays 10usd, Amazon will take it's cut like say 30 percent. Following 7usd will be divided amongst authors according to pages read. Each author will get 7 * pages read that month / total pages read that month.

For example if user reads one page of one book this month this author should get 7usd.

Medium works the same way except with reading time and maybe claps.

This at least is the most logical, fraudfree and fair way to do this.