Digital Download Requirements document

The company I work for publishes books, which we sell online, and a quarterly magazine that is sent to subscribers. We want be able to sell the books online in a digital format and offer the magazine in a digital format for the subscribers that want it vs. the printed copy.

I am going to be “in charge” of the effort to write specifications on enabling this objective. We have a draft of a digital download policy but I need something with a little more beef to start everything off.

Does anyone know where I can find a requirements document that I could work off of to help us specify how to do this?