Question: What DC Comic book did the Joker originally appear in 1940?

Context paragraphs:
DC Comics: DC Comics, Inc. is an American comic book publisher.  It is the publishing unit of DC Entertainment, a subsidiary of Warner Bros.  Entertainment, Inc., a division of Time Warner.  DC Comics is one of the largest and oldest American comic book companies, and produces material featuring numerous well-known heroic characters including Superman, Batman, Wonder Woman, Green Lantern, the Flash, Aquaman, Hawkman, and Green Arrow.  Most of their material takes place in the fictional DC Universe, which also features teams such as the Justice League, the Justice Society of America, the Suicide Squad, and the Teen Titans, and well-known villains such as the Joker, Lex Luthor, The Cheetah, Harley Quinn, Darkseid, Catwoman, Ra's al Ghul, Deathstroke, Reverse-Flash, Sinestro, Black Adam, and Brainiac.  The company has also published non-DC Universe-related material, including "Watchmen", "V for Vendetta", and many titles under their alternative imprint Vertigo.

Superman: At Earth's End: Superman: At Earth's End is a 1995 comic book Elseworlds story, published by DC Comics.  Written by Tom Veitch with art by Frank Gomez.  The story set in a post-apocalyptic future, where mutations run wild, and a white-maned Man of Steel has begun a fight against new scourge sweep across the Earth—a nightmare army of bat-men.  The book is, more or less, the sequel to the miniseries "Kamandi: At Earth's End", which itself was a reimagining of the DC Comic series "Kamandi", created by Jack Kirby in 1972.

Metropolis Collectibles: Metropolis Collectibles is a famous rare comic book dealer of vintage American comics, primarily known for its large collection of comic books originally published in the 1930s, 1940s, 1950s, 1960s and 1970s.  Metropolis was founded in 1984 by Stephen Fishler, and merged companies in 1999 with Vincent Zurzolo, Jr., of Vincent's Collectibles.; Zurzolo said that as he found he could not compete with Fishler's business, merging the two made sense.  The company is located on Broadway in New York City, and the comic book showroom allows viewings by appointment only.  Over the years, Metropolis Collectibles has grown from being a comic-book mail-order company to maintaining a major online retail presence.  In addition to being comic book buyers and comic book sellers, Metropolis also gives comic book appraisals and provides comic book valuation services of rare, old out-of-print comics.  Metropolis Collectibles has obtained a variety of notable classic comic book collections over the years, or "pedigrees", including the Crowley Collection, the Allentown Pedigree, the D-Copy Collection, and the Northford Collection.  In August 2014, the company was able to purchase a near-mint copy of "Action Comics #1" (CGC 9.0) for $3.2 million in an auction on eBay.

Justice League (Smallville): The Justice League is a fictional group of superheroes on the television series, "Smallville", who were adapted for television by Alfred Gough and Miles Millar.  The Justice League originally included Oliver Queen, Bart Allen, Victor Stone, and Arthur Curry; Clark Kent did not accept a role until three seasons later.  As the team continued to appear in the series, new characters were introduced and subsequently joined the team.  The original Justice League first appeared in the DC comic book "The Brave and the Bold" #28 (1960), and consisted of members Superman, Batman, Wonder Woman, Flash, Green Lantern, Aquaman, and the Martian Manhunter.  In "Smallville", the team did not make its first official appearance until the season six episode "Justice", although each member had been previously introduced individually on various episodes since season four.  In the series, the team never formalized a name for themselves, although the cast and crew officially recognized the team as the "Justice League".

Joker (character): The Joker is a fictional supervillain created by Bill Finger, Bob Kane, and Jerry Robinson who first appeared in the debut issue of the comic book "Batman" (April 25, 1940), published by DC Comics.  Credit for the Joker's creation is disputed; Kane and Robinson claimed responsibility for the Joker's design, while acknowledging Finger's writing contribution.  Although the Joker was planned to be killed off during his initial appearance, he was spared by editorial intervention, allowing the character to endure as the archenemy of the superhero Batman.

Jerry Grandenetti: Charles J. "Jerry" Grandenetti (April 15, 1926 – February 19, 2010) was an American comic book artist and advertising art director, best known for his work with writer-artist Will Eisner on the celebrated comics feature "The Spirit", and for his decade-and-a-half run on many DC Comics war series.  He also co-created the DC comic book "Prez" with Joe Simon.

Kid Eternity: Kid Eternity is a fictional character, a comic book superhero that premiered in "Hit Comics" #25 written by Otto Binder, drawn by Sheldon Moldoff, and published by Quality Comics in December 1942.  All of Quality's intellectual properties were sold to DC Comics in 1956 (though most of the said properties lapsed into public domain by that point), including the character.  The character has continued to appear (albeit infrequently) in DC comic books since his revival in the 1970s.

The Maxx: The Maxx is an American comic book series created by Sam Kieth and originally published monthly by Image Comics, then collected in trade paperback by DC Comic's Wildstorm imprint.  Starting in November, 2013, it has been republished by IDW with new colors and improved scans of the original artwork by Sam Kieth and Jim Sinclair.  The comic book, starring an eponymous purple hero, spawned an animated series on MTV.  The first appearance of The Maxx was in "Primer" #5, published by Comico Comics.

The Joker (comic book): The Joker is a comic book series published by DC Comics starring the super-villain the Joker.  It ran nine issues from May 1975 until October 1976.

Batman: Return of the Joker: Batman: Return of the Joker, known in Japan as Dynamite Batman (ダイナマイトバットマン , Dainamaito Battoman ) , is a 1991 platform video game, the follow-up to Sunsoft's on the Nintendo Entertainment System.  Unlike that game, which was based on the 1989 Batman film directed by Tim Burton, "Return of the Joker" is entirely self-contained and based more on the modern comic book iteration of Batman.  However, Batman mans the Batmobile from the 1989 film.  A remake of "Return of the Joker", titled Batman: Revenge of the Joker, was released on the Sega Genesis by Ringler Studios in 1992.  A Super NES version of "Revenge of the Joker" was in development, but never released.

Supporting paragraphs: The Joker (comic book) Joker (character)
Answer: Batman