This is pretty much the freemium model. Free gives you free marketing as people will be talking about it. Sharing on social etc. The pricing page is effectively

This is pretty much the freemium model. Free gives you free marketing as people will be talking about it. Sharing on social etc. The pricing page is effectively: Free: Book Premium: Book + nice feeling of helping out a nice guy And sounds like a lot of people thought the premium version was worth it. Great stuff.