N.i.n.Retro

Perhaps the most profound mystery in this level, and the game, is the sighting
of a _book_ in the 'Red Coins in a Bottle' episode. Swim down to the bottom
of the bottle and you'll find a little structure. On the left side of it a
slit large enough for Mario to creep into. Enter it and you'll find a
inoperable door. Position the camera behind the door to find the book. This
is one of the weirdest secrets in this game. No theories present as to its
purpose.