Ace Double D-286 (35 cent cover price) 1958 paperback. Condition is only about Good+: tight and square; spine has wear on edges/ends (see scan), heavy wear to bottom right of Grinnel cover (including 3? fold-traces), and light to moderate overall wear mainly as edgewear, loss of gloss and rounded corners (see scan.) Age tanning is moderate and uniform (consistent with its vintage.) Bookstore stamp on page 5 of the Silverberg side; no writing. Faint stain-trace in upper right margins of Grinnell side which fades to nothing by page 60. Finding any fifty-year-old Ace Double is rather difficult - you may wait years to see another copy. (A nice copy would be much more expensive.)

Ace Double D-286 (1958): Invaders from Earth by Robert Silverberg, bound with Across Time by Donald A. Wollheim writing as “David Grinnell.” This is the first book or first paperback appearance for either novel.

Silverberg's Invaders from Earth (cover by Ed Emshwiller, 169 pages) - expanded from magazine version in Science Fiction Quarterly February 1958 as "We, the Marauders."
Ted Kennedy, a PR man, gets the job of convincing the public that wiping out the natives of Ganymede is necessary, so that Earth can exploit the resources there. Somewhere in the middle of the project, he stops fooling himself about what he's doing, finding he has a conscience... 1 of 3 early Silverberg novels singled out by Clute & Nichols' Encyclopedia of SF as being most notable.

(Bound with) Across Time (cover by Ed Valigursky, 150 pages) - first published by Avalon Books in hardcover in 1957. From Anthony Boucher's review: "David Grinnell's ACROSS TIME is the first novel with the Grinnell by-line and rather fun… Time-traveling UFO's jerk our hero one million years into the future and launch him on a transgalactic adventure, brightened by such incidental items as an attractive post-homo race of evolved simians, and a wonderful 'Ultimate' Spaceship."
[The first page blurb]: "Chasing mysterious celestial phenomena was part of Zack Halleck's Air Force duties, so it wasn't strange that he was assigned to assist in his brother's experiment. For his scientist brother had devised a method of deliberately attracting and trapping any such sky objects. But the experiment backfired - and the Halleck's themselves were its victims. When Zack opened his eyes again, it was on the Earth of a million years in the future. And Zack learned that the only way he could rescue his brother and return to his own time would be to accept a role as a human pawn iin a conflict of galactic supermen…"