Here is an example:
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This is probably not a Powers cover, but this will give you another sense of the modernistic design of the Ballantine series:
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The following will be an updated list of these Ballantine horror paperbacks, which some online websites call Ballantine's "Chamber of Horrors" series, a term I've seen used in a FAMOUS MONSTERS ad, but one that I've yet to see in a Ballantine paperback. I'm using old FAMOUS MONSTERS magazines, the actual paperbacks, and some online hunting to generate this list.
THE GRAVEYARD READER (1958, BB 257) - ed. Groff Conklin. Twelve stories and introduction.
DEALS WITH THE DEVIL (1959, BB 326) - ed. Basil Davenport
TALES TO BE TOLD IN THE DARK (1960, BB 380) - ed. Basil Davenport
INVISIBLE MEN (1960, BB 401) - ed. Basil Davenport
ZACHERLEY'S VULTURE STEW (1960, BB 417) - ed. Zacherley
THE DOLL MAKER (1960) - Sarban
SOME OF YOUR BLOOD (1961) - Theodore Sturgeon
THE OTHER PASSENGER (1961, BB 480) - John Keir Cross. Short stories.
THINGS WITH CLAWS (1961) - ed. Whit & Hallie Burnett
ZACHERLEY'S MIDNIGHT SNACKS (1961) - ed. Zacherley
NIGHTS BLACK AGENTS (June 1961, BB 508) - Fritz Leiber
TALES OF LOVE AND HORROR (1961, BB 522) - Don Congdon
THE CLOCK STRIKES 12 (1961, BB 531) - H. R. Wakefield
SARDONICUS (1961, BB 540) - Ray Russell
NOT LONG FOR THIS WORLD (1961, BB 542) - August Derleth
ALONE BY NIGHT (1961, BB 563) - Michael & Don Congdon
RINGSTONES (1961) - Sarban
NINE HORRORS (1962) - Joseph Payne Brennan
SURVIVOR AND OTHERS (1962) - August Derleth, H.P. Lovecraft
SHADOWS WITH EYES (1962) - Fritz Leiber. Six short stories.