Unless things have changed since I was a subscriber, you get a token every month which can be used to purchase any book from the full library. So its effectively 1 book a month + a few extras bonuses for $7.95
You don't get a token without paying for the premium plan at $15 a month. Also, don't tell anyone but if you subscribe and then cancel and give the reason that it is too expensive you can often get a reduced price the next few months.