Not-So-Dead Debbie

Dead Debbie wasn't always a horrifying piece of shit. She was brutally intelligent when she underwent her cadetship with HEMLOC university, but she was a tad antisocial when it came to campus pals and roommates, nine times out of ten she'd be walled up the library either studying, reading or practicing magic and concoction-craft. 
The library is where Debbie drowned. 
Unsurprisingly, it's a case nobody insofar was able to solve, because well...how do you drown in the middle of a campus library? It's also one that Temperance and Humphrey suspect is part of a menagerie of different murders, disappearances and sightings based around a rather specific green book.