Remember back in grade school when our teachers would require that we write 20 page reports double-spaced?

We all knew that was bullshit, right? Same applies for books.

As long as the amount of value derived from the information outweighs the cost, it's a worthwhile purchase. Don't worry about the length. Write what feels natural, price it accordingly, and slap a 100% moneyback guarantee. If someone doesn't feel it was worth the price, don't take their money.