It's not anywhere in the maze tower or on the floor of the maze tower, not near the portal, not anywhere around the chest at the top or in the chest and it's not on the ground on the outside of the tower.
I've been looking for this for days. Left Soul Cairn and came back, quit game and came back, looked around again - nothing. I have to keep all the other letters in my book inventory until I finish the quest.
Does Jiub's Opus page 9 eventually reappear?