Most audio book rental services work off of the same model.  You pay a monthly membership fee, which then entitles you to receive a certain number of books each month.  You select your books directly from the thousands of offerings on the company’s website, and then they’ll ship you the title, along with a postage-paid envelope so that you can return the CDs when you’re finished.  That’s the basic structure of a membership plan, but the details vary from place to place.  For example, some companies allow you to have several titles out at one time, while others require that you go through your books one by one.  Some will charge you late fees if you are slow to return your audio book rental by a certain date, others have no such fees.  You’ll have to decide which membership features are the most important to you, then choose a service that matches your needs.