Quoth the Raven — Jane Haddam

THINGS THAT GO DEAD IN THE NIGHT
Former FBI agent Gregor Demarkian has been invited to speak at a small liberal arts college, his lecture on murder having been scheduled for Halloween night. But someone on campus has already taken the course, and is now doing graduate work in the school cafeteria...Where household lye is the soup du jour.

DEVIOUS TRICKS, DEADLY TREATS, AND THE RICTUS MASK OF MURDER...HALLOWEEN HORROR IN THE GROVES OF ACADEME!
Throw in a missing professor whom everyone hates, a raven that sings for its supper, make-believe ghouls, real-life witches, and an annual bonfire of morbid proportions, and Gregor has enough course work in Advanced Homicide 101 to fill up several Halloweens. The final exam will be pass-fail...with death giving out the final grade.