Another classic title from the Winston Science Fiction series is "Secret of the Martian Moons" by Donald A. Wollheim. This is a rip-snorting classic of science fiction from the 1950's. In the 22nd Century, mankind has colonized Mars, but they were not the first to step foot there. All across Mars, strange abandoned cities of a long vanished race beckoned to Man. Who created these cities? Where did they go? With a decent amount of action and an interesting mystery to solve, "The Secret of the Martian Moons" is well worth picking up if you can find a copy.
"Star Conquerors" so you'll just have to keep an eye out at your local used book store, thrift stores, flea markets, garage sales, ebay, or Amazon.com. The hardcover edition went through several printings (possibly as many as six) and Tempo books also reprinted it as a mass market paperback in 1963. With that many copies floating around you should be able to pick one up for a reasonable price if you keep your eye out. I hope you enjoy it as much as I do.