This is an item cut from the game, and there have been rumors of Mario needing to retrieve this book and take it to the Noki Elder, but that would make the level easier in comparision to how it is in the final version. This might not be a cut item because of being able to see it so easily. You can reach it through a glitch, but it cannot be physically touched, meaning that it is either unfinished or part of the background.
The door was probably put there to cover it up or it used to be openable, but if they intended to keep this hidden the player would be unable to use the viewer in that area. This also might be an Easter Egg.