We have a thread on whether fanfic is good or bad. Between this one and 50 Shades of Grey, I think we can say that putting aside considerations of good or bad, it is selling these days. Though in this case the only thing that makes it "fanfic" seems to be that it was originally written with members of this real band as characters. It doesn't look like it is a fanfic of another fictional work.
One Direction fanfic to become book | Shelf Life | EW.com
One Direction fanfic to become book | Shelf Life | EW.com