The Copylaw blog has a nice outline of the possible kinds of subscriptions, based on other digital subscription services:
Amazon Subscription Service Will Rewrite Book Contracts * Unlimited content model
o In exchange for a periodic fee, the subscriber receives access via streaming to unlimited content.
o Examples
+ Netflix[i]: Unlimited on-demand streaming of movies and television shows for $8/month.
+ Rhapsody[ii]: Unlimited on-demand streaming of music for $10/month.
+ Sesame Street[iii]: Unlimited e-books accessed via a computer or “widget” for $4/month.
+ Disney Digital Books[iv]: Unlimited e-books accessed via a computer or iPad app for $9/month or $80/year.
...
# Bundled content model
* In exchange for a periodic fee, the subscriber receives a pre-determined number of downloads which become the member’s to keep.
* Examples
o Audible[v]: 1 downloaded audiobook per month for $15/month or 12 downloaded audiobooks per year for $150/year; 2 downloaded audiobooks per month for $23/month or 24 downloaded audiobooks per year for $230/year.
...
# Hybrid model
* Combination of unlimited online content and either a limited number of downloads or discounted downloads.
* Examples
o Napster[vi]: Unlimited on-demand streaming music on computer and home theater devices for $4-5 per month (depending on length of commitment); adding streaming on mobile devices doubles the price. With the mobile plan, the member can save up to 100 songs on their phone for off-line listening (access to the saved songs is discontinued if the plan is cancelled).
o Safari Books[vii]: Online access to 10 books per month for $23/month or $253/year; or unlimited e-books access via a computer for $43/month or $473/year. Members also get 5 download tokens per month which can be used to download PDF versions of books.