Who cursed the Devil Fruits? One Piece is near in reaching 1000 episodes, but never in a single episode where the origin of the so called Devil Fruits is elaborated. Is there any creator of the Devil Fruits or their existence are merely phenomenal?

I remember one episode when Blackbeard killed one of his comrades just to gain the Dark-Dark fruit, he was sure that based on its appearance, it was definitely the Yami Yami no Mi because he read it in the book about the cursed fruits.

How was the book written, does it imply that the author of the book is also the one who cursed the Devil Fruits?
Oda mentioned in SBS vol 48, that it will be revealed in the end by someone who knows how the Devil Fruit mechanism works. Quoting from the wikia:


  According to Oda, a "certain professor" will explain the complex mechanisms behind all of the Devil Fruits in the series in the near future.


And I personally think all of your other questions, like "regarding the author of the book", will be answered when that happens.