Yup, there's some controversy over whether to ever make any of your books free. Well, it's a personal choice and a good marketing strategy for those authors, myself included, who have written a series.
The hope is that readers will be sufficiently interested after downloading the first one for free to then go on and buy the next instalments.
I have now made my first book, The Awakening, permafree on all digital platforms - including Amazon.
Another opportunity is that this marketing option might lead to more reviews, which admittedly doesn't improve one's sales rankings, but more reviews mean a better purchasing experience for the consumer.