Ace Double D-358: The Plot Against Earth by Robert Silverberg writing as "Calvin M. Knox," bound with Recruit for Andromeda by Milton Lesser. (35 cent cover price) 1959 paperback. This is the first paperback appearance for either novel.
The Plot Against Earth - Cover by Ed Valigursky; 138 pages. Here's the first-page blurb:
NEVER FOLLOW A FALLING STAR!
The humanoid worlds of the galaxy were alarmed! Somehow, somewhere the mind-destroying hypnojewels were being trafficked in.
An uneasy Earth, newcomer to the ranks of the civilized planetss, sent Lloyd Catton to the Interworld Crime Commission of Morilar to investigate. Although the Commission had made little progress until then, after his arrival things began to happen fast.
For it didn't take Catton long to realize that the hypnojewels were but the thin edge of a murderous wedge that was calculated to shove the Earth back into the helpless isolation of a world returned to savagery.
(Bound with) Recruit for Andromeda - Cover by Ed Emshwiller; 117 pages. Magazine version as "Voyage to Eternity" in Imagination July 1953. Here's the first-page blurb:
TOURNAMENT UNDER NIGHTMARE SKIES
When Kit Temple was drafted for the Nowhere Journey, he figured that he'd left his home, his girl, and the Earth for good. For though those called were always promised "rotation," not a man had ever returned from that mysterious flight into the unknown.
Kit's fellow-draftee Arkalion, the young man with the strange, old-man eyes, seemed to know more than he should. So when Kit twisted the tail of fate and followed Arkalion to the ends of space and time, he found the secret behind "Nowhere" and a personal challenge upon which the entire future of Earth depended.