Info about the .mobi file format can be found here

More information here (Quite detailed).

Also … here

IDEA

Why not use the iPhone as a payment method. i.e. buying the app gives you access to a service - nothing more.

Use the source code of Calibre.

GAE based ?

Is iOS the best platform to go with? Not sure but it does provide us with an easy payment method. If we go primarily web based then can port to Android / WebOS.