I was wondering where I left that.
I wonder what that label says....
In Super Mario Sunshine at Red Coins in a Bottle in Noki Bay, there is a rock formation at the bottom, and a small entrance with a partly open door, with a Secret Book inside it.
Cut Item[]
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 comparison 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 able to be opened, 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.
Rumors[]
As stated previously above there have been many rumors on the books origin and also possible ways to acquire the book. Most, if not all of these theories have been proven false, thus its believed that there is no way to acquire the book. There are some that believe that in the Japanese version the Red Coin mission is instead a mission where you are to retrieve the book and bring it back to the Noki Elder, but this is not confirmed.
A fake image of someone acquiring the book.