Not good, friends, not good.
I picked up this 1965 Ace Double because I had heard of The Alternate Martians, a story where space travelers find themselves on a fictional-consensus Mars that combines famous past visions of the planet. This is accurate, that is indeed what happens in the novel, but I found that the execution fell far, far short of the concept, being a generic adventure on a No Name brand Mars that doesn't really do much but replay some ideas from Wells and Burroughs. Now I will say that for all I know the author was the pioneer of what I now take as accepted wisdom (the interesting idea that Wells' "Martians" aren't really from Mars, the botched terraforming of the red planet, etc.), but it just wasn't enough to get me engrossed in the story. I'll add to that the fact that this is transparently a sequel to something—I have no idea what because it's not actually stated anywhere. I only figured out it was a sequel because the early stages of the novel describe recent events in the protagonist's life in startling detail, and I started to think oh, this must be another novel he's talking about. What I would say is that you should read Rainbow Mars by Larry Niven, which is a very entertaining book about a visit to a Mars where all the bizarre old-fashioned fiction is true.
The other half of this double novel is Empress of Outer Space, which I also didn't enjoy very much at all. Oddly it also involves a detour into a fantasy world based on old-fashioned adventure fiction, but I found it extremely precious instead of interesting, and once again, it's just an off-the-shelf plot with generic characters. The basic concept of this universe is interesting, as the title character is a relatively ordinary woman who has been made absolute Empress of a sprawling space empire for a limited term, and she takes command of a ship in order to pursue a pirate who has stolen her yacht. If you're wondering about the government described here, don't worry, a helpful character will take the time to "as you know" another character about the government they have both lived in their entire lives and illuminate everything. Anyway, she's quite headstrong as you would expect the Empress of outer space who is also a Master Astronaut to be, and she and our protagonist have kind of a bickering relationship. If you can figure out what happens in the last chapter between the imperious ice queen and our long-suffering but capable hero, then I suppose you have ever read this kind of thing.
I do not recommend either of these novels, but because they are competent and interesting ideas appear though they're not handled well, I've given it two stars—losing points for a variety of classic SF sins. Yes, I am still mad about the "as you know" speech.