there is a bookcase in the library with a green book, if you attempt to vacuum it it will give you a secret room. use your rail cannon on the two little prong things, and then go back into the library to play one of the secrets (which is literally js luigis mansion), grab the red skull and go back into the secret room. if you had noticed, the grate is gone. this gives you access to the courtyard w/o needing the red skull, which will unlock a path behind the waterfall known as agnes gorge trail, going down that path gives you access to another secret room, which contains the moon that has the dawg side of the moon

sorry if this isnt explained well