It is interesting just how much of The Door Through Space Bradley cribbed for her Darkover series: The Terran Empire colonizing the universe, a world bound by compact rather than charter to the Terrans, culture clash, Dry Towns, The Ghost Wind, a hint of ESP, chains binding women, the red sun, catmen, and the exclamation of the word "Sharra". These are both superficial as well as deeply thematic similarities to Darkover. In truth, if The Door Through Space was only given minor edits, it could pass as a Darkover novel. This was Bradley's first published novel and Darkover was obviously a work in progress throughout her entire career, but it is interesting to note how much of this novel she used to create an entire series of novels completely unrelated to this one. The world of Wolf could easily be Cottman IV. Had the word "matrix" shown up anywhere in TDTS I would have cried foul.
Race Cargill is a Terran intelligence agent who has been stuck behind a desk because of a bitter dispute with another agent who has "gone native". When Cargill's sister comes to Race because her husband, the former friend and agent who maimed Cargill, has apparently threatened her and her daughter, Cargill goes back into the field instead of leaving the planet for good. Adventure ensues.
Honestly, the book isn't that good. It is a pulpy science fiction and fantasy blend that works less well than any of her later, more developed Darkover novels. Add to the fact that having read the majority of Darkover, The Door Through Space comes off as a cheap copy, no matter that this book came first. It is a weaker Darkover novel without any of the trappings that make Darkover compelling. It is as if Bradley were trying out the ideas which would later mark her as a top talent in the 1970's and 1980's. The novel is short enough, which is good, because 300 pages of this would be rough going. The novel is not all bad and there are positives in her description of the customs and traditions of the cultures she introduces. Her handling of character, however, is less skillful.
Overall, no need to read this. Science fiction has been done far better, and Bradley herself would later re-write this novel into the vastly superior Darkover series.