subscription
A funding model based on a recurring monthly fee.
Typically, the fee is paid in exchange for access to some service. (If there is no specific service given in return, the subscription is a form of patronage or micropatronage.)
In the context of e-publishing, the subscription service might be early access to new works by the author, the ability to communicate more directly with the author, free access to a large library of books, or any other benefit which is accessed repeatedly by the subscriber.
Also see: crowdfunding, patronage, micropatronage, donation, pay-what-you-want.