Right now the problem with Amazon Unlimited as far as book selection goes is they have to be exclusive to Amazon to be part of Unlimited & people making a decent sum with B&N, iBooks and others are naturally unwilling to participate.
But I've blown through 9 books (two whole series) in 4 days, which I've never been able to do with Overdrive due to waitlists & lack of selection & not being able to have more than 2 books out at a time. So this free trial is a total win for me to this point.