I have "The Door Through Space" as the red half of Ace Double F-117 (do Ace Doubles rule, or what?). List price, forty cents. Cover banner "Beware of the 4-D Demons", which reflects the actual plot not at all.
It's a blast from the past in various ways besides the packaging. It's solid swashbuckling spaceopera SF from a time when good writers didn't shy away from writing swashbuckling spaceopera SF. It's the novel that MZB wrote when Darkover was still a gleam in her eye; the setting (a planet called Wolf) has Darkover's Dry Towns with their manacled women and subtle codes of honor, but it also has mysterious telekinetic aliens and a lurking Frog-god (at least I don't remember a Frog-god on Darkover). It takes for granted that there are two kinds of humans on Wolf: the ones from Earth, and the "Wolfians"; on Darkover, Bradley explained how that happened.
(The name "Darkover" in fact occurs once in the book, in a passing reference to some horses that came from there. The name must have grown on her.)
As a narrative it's adventurous and fun. I didn't find the ending terribly satisfying (bit of a deus ex), but endings are hard. And it's not just blasters and aliens; there are some interesting themes lurking. Cultural rivalry and dominance, the lure of the new and loyalty to the ancient, different kinds of chains.
I found a Web page describing Bradley in a 1976 interview where she "[t]alks about Door Through Space and male hero there being her animus, and image of herself as chained woman. Took her twenty years to recognize that self-portrait and send a group of women to free her".
The last line of the book complements that in an odd sort of way. "I vowed to myself as we went that I should waste no time finding a fetter shop and having forged therein the perfect steel chains that should bind my love's wrists to my key forever."

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