E. Gadd's Guide to Ghosts is a book written by Professor Elvin Gadd himself and found by Luigi in the Study of the mansion. It describes the difficulty of catching of uneasily surprised ghosts. He tells the reader to catch them off guard and unsettle the ghosts. He then says that the reader should start catching the ghost(s) only once it has revealed its heart.
Guide[]
"Dear readers... We've all had trouble with ghosts who aren't easily surprised, right? They have all sorts of ways to protect themselves from light. What would you do, bright folks, when in a fix with one of these stubborn ghouls? Well sir, I'd first look for an opening... ...A way to catch them off guard and unsettle them. When a ghost is surprised and shows you its heart, that's your chance to catch the little bugger. I tell you, I've caught dozens of ghosts just like this..."