Impact-20 (1963) - William F. Nolan's first collection of stories. Eight of these can be found in later Nolan collections - which leaves 12 only found here.

(Introduction by Ray Bradbury)
The Small World of Lewis Stillman - (original title= "Small World")
The Beautiful Doll Caper
Into the Lion's Den - (original title= "The Strange Case of Mr. Pruyn")
The Martians and the Leadfoot
The End With No Perhaps
The Amazon Kick
Full Quota
And Miles to Go Before I Sleep
The Public Loves a Johnny
Dutch
Of Time and Texas
Death Double - (a.k.a. "The Stuntman" or "Stuntman")
Dark Encounter
The Lap of the Primitive - (a.k.a. "Tildy")
Nobody, That's Who
The Darendinger Build–Up
Pigeon - (original title= "To Kill a Pigeon")
The Joy of Living
One of Those Days
To Serve the Ship - (a.k.a. "Solution")

"The End With No Perhaps", "The Public Loves a Johnny", "Death Double" and "Nobody, That's Who" may be new to the 1963 paperback - or may have appeared in non-SF sources I'm unable to trace. Most of these originally appeared from 1956 to 1959 [with one from 1954 & one from 1963]: in Fantastic Universe (4), Nugget, Alfred Hitchcock's, West Coast Sports Car Journal, Rogue, Infinity, The Saint (2), Playboy, Mike Shayne's, IF, Magazine of F&SF, Galmma.