Coupons for access to members area of website.

Hi guys

I’m hoping someone can help me, I’m helping out someone who is an author. They would like to publish there book online and provide whoever buys the book with access to a members only area on there website.

So I was wondering if anyone knows of a good coupon plugin which I can use to grant access to specific areas of the website using that code provided. I still haven’t decided if I should just issue one code that works continuously or if I should create a unique code for each book, that all depends on the author so if I could get hold of a plugin that will provide me the option of both that would be great.

Thank you for your time