"Empress of Outer Space" is a 1965 science-fiction novel by A. Bertram Chandler, published as one-half of an Ace Double along with The Alternate Martians, also by Chandler. The two books are related thematically, but are apparently set in different continuities. Chandler had many separate continuities that shared some key features, usually being shipboard space operas. I also read the third book in this sequence, Nebula Alert, previously.
The book seemingly starts in media res, (despite being the first book in the series?) with the aforementioned Empress of Outer Space and one of her captains going off to hunt down an usurper who has stolen a spaceship. (The book gives a brief "As You Know, Bob" sequence as to why we have a Space Empress). While doing so, in a case of mistaken identity, another spaceship fires at them, and they come down on an uninhabited planet. The crew then steps into some hallucinogenic plants (with many direct references to The Wizard of Oz) and they all fall into a shared hallucingenic dream sharing aspects of The Wizard of Oz, James Bond and the works of Edgar Rice Burroughs, and also a daring escape from a crashing Zeppelin. This gives "Empress of Outer Space" commonalities with the reverse story, The Alternate Martians, as we go through a mish-mash of reference to past science-fiction legends. At the end, we wake up to find it was all just a dream, besides their shared dream adventure in a crashing blimp now makes the Empress and our protagonist fall in love and she abdicates and they ride off into the sunset.
I found this book a little surprising, because A. Bertram Chandler generally writes well-done, but obvious space opera based on relatively realistic social structures and physics. This book started out in that mold, but the latter half of it was a fever dream mish-mash of different concepts and genres thrown together, in a type of ironic metafiction. Which was a good departure from the predictable, but as is often the case, I wish it could have been developed better.