And here's how the book ends.... (look away if you're planning on reading it).
So, Delaney, the beautiful scheming student is dead. So is Jesse, another student. Jesse's father, the head of the history faculty, is grief-stricken. The wrong man is promptly arrested.
A copy of a book by an obscure French author called Constant was found on Jesse's body (hence the title. ho ho ho). This was mentioned but then forgotten about.
Nick, our hero, finally reads the back cover blurb of the book and solves the mystery. It is about a sinister young man who seduces an older woman. He realises that Delaney killed Jesse, because Jesse had discovered that Delaney drove his mother to suicide. Nick goes off to confront Jesse's father who admits that, yes, he was the murderer. And he has Delaney's appallingly unsettling diaries. He would like Nick to go home and read them and then decide what to do with them.
So.... Nick knows who has committed two murders, and holds VITAL evidence which could free an innocent man, and bring down the entire campus administration.
Here's what happens next (with page numbers)
262: Nick gets home. Stefan is cooking. They talk about French literature
263: Nick explains the plot of the Constant book.
264: Stefan says Nick shouldn't have confronted the murderer on his own.
265: They start to read the diary. "We broke off when the timer rang. We ate wild mushroom lasagne...."
266: They load the dishwasher. "The quiet chugging seemed to mirror Stefan's musing..."
267: They read more of the diary. Then they talk about an offer Nick's received to edit a book.
268: They talk more about Nick's career prospects on campus.
269: They remember that an innocent man is in jail. They leave a voicemail with the police, watch a movie, and go to bed.