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Halloween Party by Agatha Christie
Witches Bane by Susan Wittig Albert
The Hunt Ball by Rita Mae Brown
Sympathy For The Devil by Jerrilyn Farmer
A Few Dying Words by Paula Gosling
The Fallen Man by Tony Hillerman
Long Time No See by Susan Isaacs
Tricks: an 87th Precinct Mystery by Ed McBain
A Hole in Juan by Gillian Roberts
A Graveyard for Lunatics by Ray Bradbury
Death on Allhallowe'en by Leo Bruce
Ghostly Murders by P. C. Doherty
Trick or Treat by Leslie Meier
The Body in the Moonlight by Katherine Hall Page
Strange Brew by Kathy Hogan Trochek
All Hallow's Eve by Charles Williams
All Hallow's Evil by Valerie Wolzien
Short story mavens don't worry: Here's a list of Halloween Mystery Short Stories:
Trick and Treats edited by Joe Gores & Bill Pronzini
Asking for the Moon (includes "Pascoe's Ghost" and "Dalziel's Ghost") by Reginald Hill
Murder for Halloween by Cynthia Manson
The Haunted Hour, edited by Cynthia Manson & Constance Scarborough
Murder for Halloween: Tales of Suspense, edited by Michele Slung & Roland Hartman.
Mystery for Halloween (an anthology), edited by Donald Westlake
Not enough? Get out your Edgar Allen Poe and read "The Telltale Heart". Not quite Halloween, but in the spirit.
Boo!