Someone please tell me how to get to the third story of Big Boo's Haunt..

As Mario, gain access to the second floor and find the room with the bookcase that emits mutant books. At the end of the galley you should see a large shelf and a ! box. Burst the box to transform into a balloon and float up to the landing above. Go through the door and jump into the portrait of Luigi; you should come into an atrium with collapsible bridges. The objective is to progress through these atria by seeking out the door that will take you into the next atrium. Eventually you will enter a room with a large mirror from which Mario will reflect as Luigi. To free him, you must defeat the invisible Boo; to locate it and avoid its attacks, keep track of it in the mirror or watch the shadow. It will reappear when your back is turned; pound it until it evanesces and you should obtain the key to Luigi's dormitory.

Ok, I figured this out on my own.

In the second story there is a bookcase shaped like a V in a room with a tilty floor panel. There is a elevated area near the end of shelves with a ? Block on it. Either get Luigi's cap and do a backwards somersalt up, or stay as Mario and float up using a power flower. You need to beat Bowser in Lava Sea, which is in front of the Dire, Dire Docks portal, to get to the mirror room. He'll give you the key to the third story. Go through the locked door on top of the main staircase. At the top of the staircase, you need to go through one of the doors into a room filled with mirrors.