The Defiant Agents by Andre Norton

(Time Traders #3) 1978 ACE 6th printing in very good condition. No stress lines. Nice tight copy with some very mild edgewear. 
 
Travis Fox, Apache Indian and an expert in time travel, volunteers for a mission to colonize the planet of Topaz. But when he and his fellow Apaches find themselves reverting to the ways of their ancient warrior race, Fox suspects that a dangerous force is responsible--and that there's more at stake than just their lives.