I admit that I had at first thought it to be Rumpelstiltskin, then switched to August when he worked on the book. In all honesty, I think it is none of the above. August has been confirmed to have not written the book, I doubt Regina would have written it, seeing as how it reveals almost all the true stories of the fairytale characters, and Jefferson has not been given a hint to have written it–plus I doubt he knows all the stories in the book. And he would have used the book to convince Emma when he first met her.
The Queen of Hearts has not been seen much–still doubtful that she wrote the book. There’s never been a hint of her leaving Wonderland, other than the curse appearing. It could have been Rumpelstiltskin, but I don’t see much gain for him, nor do I think he would have seen how much needed the books is. The book helped Henry to believe, but Rumpelstiltskin couldn’t have known that Emma would have a son (the writers have confirmed that he didn’t remember until he heard Emma’s name) so he wouldn’t have been able to keep an eye on, and it doesn’t seem possible that he would have been able to set her up with someone.
As seen in The Return, Bae goes through the portal, so I doubt he was able to write the book. He might have known some of the stories–it is never clearly stated how long he stayed with his father after he became the Dark One. The Blue Fairy is a possibility–but then why not include stories of Rumpelstiltskin? And how would she have known about the Mad Hatter and Wonderland? Can the Fairies enter Wonderland?
I admit that it would have been a little cool if Belle wrote the book–a great nod to both the oringial fairytale and the Disney film. but that also seems unlikely. True it is unknown how long Belle stayed with him, but I doubt it was long enough to see the other Fairytale characters, and nor is it certain that Rumpelstiltskin told her about how dealings–it is possible, though.