An ABC of Science Fiction, a reprint-anthology edited by Tom Boardman (1966.) The main gimmick is obvious: a story by an author for each letter of the alphabet. Tom Boardman in his introduction explains the less obvious intent - to show stories "that range over the whole spectrum of SF: pasts, presents, and futures; gadgets and inventions; men and supermen; robots and aliens; time travel and psionics... an ABC of ideas as well." I think he has succeeded: many of these are uncommon unless you have a huge magazine collection, and some are quite good - I particularly recommend the Aldiss story at the beginning (not in any Aldiss collection.) Original appearances from 1809 to 1966 - in Science Fantasy, Future SF, Magazine of F&SF (8), Galaxy (4), Astounding (3), Amazing, Worlds of Tomorrow (2), Fantastic (2), Space SF, Angels and Spaceships, History of New York.

(Introduction by Tom Boardman, Jr.)
"Let’s Be Frank" by Brian W. Aldiss
"Pattern" by Fredric Brown
"The Awakening" by Arthur C. Clarke
"I Do Not Hear You, Sir" by Avram Davidson
"Day at the Beach" by Carol Emshwiller
"The King of the Beasts" by Philip José Farmer
"Homey Atmosphere" by Daniel F. Galouye
"Mute Milton" by Harry Harrison
"The Conquest of the Moon" by Washington Irving
"In the Bag" by Laurence M. Janifer
"Maid to Measure" by Damon Knight
"X Marks the Pedwalk" by Fritz Leiber
"No Moon for Me" by Walter M. Miller, Jr.
"Family Resemblance" by Alan E. Nourse
"Final Exam" by Chad Oliver
"The Bitterest Pill" by Frederik Pohl
"He Had a Big Heart" by Frank Quattrocchi
"Love Story" by Eric Frank Russell
"The Fence" by Clifford Simak
"Project Hush" by William Tenn
"The Finer Breed" by Helen M. Urban
"Harrison Bergeron" by Kurt Vonnegut, Jr.
"Close Behind Him" by John Wyndham
"Three Limericks" by B. T. H. Xerxes
"Thirty Days Had September" by Robert F. Young
"The Great Slow Kings" by Roger Zelazny