Looks like a good deal of their strength is based on screen adaptations, though I'm kind of surprised to see Gone Girl still so high. There's a lot of buzz about Unbroken and Wild so those don't surprise me. The Fault in our Stars and Outlander are still hanging in there further down the list.
#15 is interesting. Didn't realize self-published werewolf porn was so popular, but for 99 cents, I guess you can sell a lot of copies.