Gregory Macguire's Out Of Oz being the final chapter in The Wicked Years series, there were some questions still left unanswered. Most of these concerned the Grimmerie itself, but I've tried to narrow down the bullet points of each area left uncovered by the books without trying to lead to much into personal head-canon.
*Glinda's possession of the Grimmerie
It's established that at some point after Elphaba's death, Glinda had somehow come into possessing the Grimmerie. Later on, Mr. Boss, the dwarf who operated the Clock of the Time Dragon, had relieved her of it, and then early in Out Of Oz gives it back to Glinda so she could use its magic to freeze Loyal Oz's dragon navy. But exactly how the Grimmerie went from Kiamo Ko to wherever Glinda was never explained. If Glinda went to the castle and retrieved the book on her own, or if Mr. Boss accosted it and brought it to her in the first place is uncovered.
*Glinda's memory loss
It is possible that Glinda used a spell on herself to make her forget originally keeping the Grimmerie, as well as taking in Rain and not remembering that she was Elphaba's granddaughter would fill in a little of the gap of the first point. But, nothing is shown about the Grimmerie being able to effect people's memories.
*Mr. Boss' boss
Mr. Boss' mission in Oz was to protect the Grimmerie, of which he used the Clock of the Time Dragon to occasionally house the book. But the dwarf claimed that he left his own world(supposedly Earth)to go to Oz and carry our the wishes of his own employer. Who this Boss of Bosses was never elaborated. It might have been the wizard that was said to have first brought the Grimmerie to Oz in the first place, whose job was to keep it safe from someone or something in our world.
*The purpose of the Grimmerie in Oz
The tome of magic was sort of Maguire's interpretation of Glinda's Great Book of Records which in the original Oz books was a large book that gave a brief history of the entire world from the dawn of time to modern day and could say what was happening at that exact minute, but not the future. The Grimmerie itself was more of a living book of spells capable of manifesting creations on its own, such as Mother Yackle. It's unclear if Yackle was a person that lived in Oz that was given a black slate on their memory(who might have been Elphaba's grandmother/grandaunt), or if she was created by the Grimmerie itself as she was folded into it at the end of A Lion Among Men isn't told. But the Grimmerie itself supposedly came from our world and was sent to Oz in order to hide it. The wizard who brought it there, who was not Oscar Diggs, either took it to Oz on his own, or was part of some organization with the task of guarding it. Oscar Diggs came to Oz at first to find the Grimmerie, but if this was something he did on his own or if he had any affiliation with this "Grimmerie Protection Society" remains a mystery.
Whatever happened to the Scarecrow and Tin Woodman
After Dorothy and her travelling companions returned to the Emerald City, we never learned much of what became of the Tin Man other than he went on to start some kind of "tik tok" revolution. Aside from Grommetik, we never learn about any other automatons in Oz which were supposed to be a minor work force under the Wizard's rule. Why they would need a revolution unless they somehow gained sentience and didn't want to be exploited any more is bizarre, that and why a non-tik tok like the Tin Woodman would like to lead the strange cause is very murky. As for the Scarecrow, we do know that after a final rendezvous with Liir after coming back to the EC, he was made into the Throne Minister after Glinda stepped down. It been established that there was a replacement "straw man" for the Scarecrow to replace him, mostly for public appearances while the shift was made for Shell Thropp to become Emperor of Oz. If that stand-in Scarecrow was the original Scarecrow with his straw stuffed into a fresh sack doesn't come up, and if so what happened to the first Scarecrow, or whether he was ever Fiyero reincarnated into a new body as in the musical.