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.