Unique Coupon for free product?

We want to open a shop for customer can purchase and download books (no shipment fees).

Since we are in the health service, we would like offert the possiblity to purchase 10 or 12 books on a period of 1 year, so people can select a book related on their current health situation at any moment of the year.

I didn't find nothing at this time on the web which allow an amount of download by purchase (limit on a period of time not from 1 order).

But after read about the coupon system, does it could be possible that someone purchase a plan (which allow him to have 12 books at a lowest cost) and he receive coupons codes exclusive for him (unique, mean work one time each, or 1 coupon working 12 times only), and he only have to use this coupon with the products he want to purchase later and the item will be free with the coupon?

Or any other idea about how I could succesfully this kind of service?

Thank you for your help.

• Lea