I've noticed something regarding the Kindle First books.
If you haven't heard about Kindle First, it's a program Amazon has where their editors select four new books each month. The books are available in advance of their official release date. Prime members can choose one of the four books for free every month. You can sign up for email reminders so you know when the books are available.
So anyhow, I've noticed on the Kindle First books, once they are released the next month, most if not all of them are free to read with Amazon Prime (one free per month). So if you are ever interested in more than one and don't want to buy them, and have Prime, you can still read them for free.