Categories:
1. Colorful Crime: A book with a color or reference to color in the title
2. Murder by the Numbers: A book with a number or quantity in the title
3. Amateur Night: A book with a detective who is not a P.I.; Police Officer; or other official investigator (Nurse Keate, Father Brown, Miss Marple, etc.)
Eleven Came Back by Mabel Seeley, 1943, in which our narrator/protagonist, Martha Chappell, assumes the role of detective.
4. Leave It to the Professionals: A book featuring cops, private eyes, secret service, professional spies, etc.
5. Jolly Old England: A mystery set in the United Kingdom
6. Yankee Doodle Dandy: A mystery set in the United States
7. World Traveler: A mystery set in any country except the U.S. or U.K.
8. Dangerous Beasts: A book with an animal in the title
9. A Calendar of Crime: A mystery with a date/holiday/year/month/etc. in the title
10. Wicked Women: A book with a woman in the title--either by name (Mrs. McGinty's Dead) or by reference (The Case of the Vagabound Virgin)
11. Malicious Men: A book with a man in the title--either by name (Maigret & the Yellow Dog) or by reference (The Case of the Haunted Husband)
12. Murderous Methods: A book with a means of death in the title (The Noose, 5 Bullets, Deadly Nightshade, etc.)
13. Staging the Crime: A mystery set in the entertainment world (theatre, musical event, pageant, Hollywood, etc)
14. Scene of the Crime: A book with the location of the crime in the title
15. Cops & Robbers: A book that features a theft rather than murder
Crooked Lightning by Erle Stanley Gardner, 1928 (jewel theft)
16. Locked Rooms/Impossible Crimes: A locked-room or otherwise impossible crime mystery (locks not necessary).
17. First Impressions: Randomly select four books from your TBR pile. Read only the first line of each book and select one of them to read based on your first impression of the book.
18. Country House Criminals: A standard (or not-so-standard) Golden Age-style country house murder
19. Murder on the High Seas: A mystery involving water
20. Planes, Trains, & Automobiles: A book with a mode of transportation in the title
21. Murder is Academic: A mystery involving a scholar, teacher, librarian, etc. OR set at a school, university, library, etc.
22. Things That Go Bump in the Night: A book with something spooky, creepy, gothic in the title (The Skeleton in the Clock; Haunted Lady; The Bat; etc)
23. Repeat Offenders: A mystery featuring your favorite series detective or by your favorite author or reread an old favorite
24. The Butler Did It...Or Not: A mystery where the butler is the victim, the sleuth...(gasp) the criminal...or is just downright memorable for whatever reason.
The Crime Conductor by Philip MacDonald, 1931, in which the butler Vickers has a secret past which is part of the plot.
25. A Mystery by Any Other Name: Any book that has been published under more than one title (Murder Is Easy--aka Easy to Kill [Christie]; Fog of Doubt--aka London Particular [Christianna Brand], etc.)
26. Dynamic Duos: A mystery featuring a detective team (Holmes & Watson; Pam & Jerry North; Nero Wolfe & Archie Goodwin, or a little-known team that you introduce to us)
27. Size Matters: A book with a size or measurement in the title (Death Has a Small Voice; The Big Four; The Weight of the Evidence; etc.)
28. Psychic Phenomena: A mystery featuring a seance, medium, hypnotism, or other psychic or "supernatural" characters/events
29. Book to Movie: A book that has appeared on screen (feature film or TV)
30. The Old Bailey: A courtroom drama mystery OR a mystery featuring a judge, lawyer, barrister, district attorney
31. Serial Killers: Books that were originally published in serial format (from the pulp era) OR a book that includes three or more deaths--all committed by the same person.
The Mystery of Marie Rogêt by Edgar Allan Poe, 1842, published as a newspaper serial
32. Killed in Translation: A work that originally appeared in another language and has been made available in English--original publication date determines Gold or Silver Age--OR if your native language is not English, then a work that originally appeared in English which you read in your native language.The Horla by Guy de Maupassant, 1887
33. History Mysteries: The bulk or focus of the mystery must take place at least 15 years prior to the date of publication. Flashback stories are fine as long as the modern events frame the story OR the historical events are absolutely vital to the present day story. Example: A Study in Scarlet by Sir Artur Conan Doyle.
The Crouching Beast by Valentine Williams (1928) in which the story takes place just prior to World War I.
34. International Detectives: A variation on "World Traveler"--but instead of the crime being set in another country, the detective is not from the U.S. or U.K.
The Big Shot by Frank L. Packard, 1929, in which our amateur detective is a Canadian woman.
35. Somebody Else's Crime: Read a book that someone else has already read for the challenge.
36. Genuine Fakes: Read a book by an author who wrote under a pseudonym (Josephine Tey [Elizabeth Mackintosh]; Nicholas Blake [Cecil Day Lewis]; etc.)
37. Hobbies Can Be Murder: A mystery that involves a hobby in some way: stamp, coin, book collecting, etc; knitting; birdwatching; hunting; etc.
38. Snatch & Grab: Read the first book you pick up off your shelf or TBR stack/s
39. I've Got You Covered: Pick a book to read based on the cover
40. Get Out of Jail Free: One per customer. You tell me what special category the book fits and it counts--the only thing I won't accept is "It's a vintage mystery!" The genre/time period is a given.
Duty Free by Manning Coles, 1959. Submitted in the literal sense - It features one character who escapes from jail successfully. 
