The book is rather good, though it is mainly devoted to science fiction work, with only a few repros of his horror covers
It does have a checklist of Powers covers though, and there are masses of them...from this I've culled the following list of Ballantine and non-Ballantine horror titles. In many cases where I know nowt about the books in question I've had to make a guess on whether they're horror titles or SF; I've tried to err on the side of inclusiveness but the fact that some people e.g. Henry Kuttner and Fritz Leiber wrote both isn't exactly helpful!
Ballantine (probable) horror titles: David Duncan, Dark Dominion (1953)
Gerald Kersh, The Secret Masters (1953)
John Wyndham, Tales of Gooseflesh and Laughter (1956) [reissued 1966]
Groff Conklin, The Graveyard Reader (1958)
Edmund Cooper, Deadly Image (1958)
Gerald Kersh, On an Odd Note (1958)
Basil Davenport, Deals with the Devil (1959) [reissued 1966]
Basil Davenport, Invisible Men (1960) [reissued 1966]
Basil Davenport, Tales to be Told in the Dark (1960) [reissued 1966]
Zacherly, Zacherly's Midnight Snacks (1960) [reissued 1961]
Zacherly, Zacherly's Vulture Stew (1960) [reissued 1961]
Whit & Hallie Burnett, Things with Claws (1961) [reissued 1966]
Michael & Don Congdon, Alone By Night (1961) [reissued 1967]
Don Congdon, Tales of Love and Horror (1961)
John Keir Cross, The Other Passenger (1961)
Fritz Leiber, Night's Black Agents (1961)
Fritz Leiber, Shadows With Eyes (1961)
Ray Russell, Sardonicus (1961)
H.R. Wakefield, Stories from the Clock Strikes 12 (1961) [reissued same year]
Charles Beaumont, The Fiend in You (1962)
Calvin Beck, The Frankenstein Reader (1962)
James Blish, Night Shapes (1962)
Joseph Payne Brennan, Nine Horrors and a Dream (1962)
H.P. Lovecraft, The Survivor and Others (1962)
Ray Bradbury, The October Country (1964)
Lester Del Ray, Mortals and Monsters (1965)
Non-Ballantine (probable) horror titles: H.P. Lovecraft, Cry Horror! (1958)
Richard Matheson, A Stir of Echoes (1959)
Theodore Sturgeon, A Touch of Strange (1959)
Murray Leinster, Creatures of the Abyss (1961)
Richard Matheson, Shock! (1961)
Stewart Beach, This Week's Stories of Mystery and Suspense (1962)
Hal Cantor, Ghosts and Things (1962)
Shirley Jackson, The Sundial (1962)
Gerald Kersh, Men Without Bones (1962)
Fritz Leiber, Gather Darkness (1962)
Murray Leinster, Operation Terror (1962)
Dorothy L. Sayers, Stories of the Supernatural (1963) [reissued 1965]
M.P. Shiel, The Purple Cloud (1963)
Peter Saxon, The Torturer (1970)
Fritz Leiber, Our Lady of Darkness (1978)
It's said in the main text of the book that Conklin's
Graveyard Reader was the first in the Chamber of Horrors series.
I notice that Theodore Sturgeon's
Some of Your Blood, which forms part of the montage at the top of the thread and so presumably is a Chamber of Horrors title, isn't included in the checklist. So either the list has omissions (very possible) or maybe Powers didn't handle the entire series...?