Copyright: This work is in the public domain in the USA only.

Ace "Double" book. Covers by Robert E. Schulz , Stanley Meltzoff. "Sentinels of Space" (aka "Sentinels from Space") was first published in the November 1951 issue of Startling Stories magazine. The reverse side is an anthology of stories: The Ultimate Invader by Eric Frank Russell (aka Design for Great-Day 1953); Alien Envoy (1944) by Malcolm Jameson; Malignant Marauder by Murray Leinster (aka Dead City 1946); The Temporal Transgressor by Frank Belknap Long (aka Bridgehead 1944).