This way there's no cost to you because your customer has paid for your second manual, so you're in business for free. When your prospective customers 30 days are up, you send them a reminder to pay their money in order to get the next manual. And most likely they are doing the same with their potential customers. By now there could be hundreds of people, all waiting for their prospects to pay for the next manual. Everyone is waiting to get that first payment, but it never happens.
Nobody wants to make that first initial payment out of pocket.
You can't sell the contents of the free manual because it is copyrighted material, nobody wants to purchase the next manual out of their own pocket, and now you won't send for the second manual because you think it's not a good seller because nobody's ordering from you. You think you'll be just wasting your time and money on another worthless program. So there's nothing left but to trash the project and move on to something else. Fact is, people are dreaming if they hope to get a profitable business going for free. Anything worthwhile involves some sort of investment.