"The Alternate Martians" is a 1965 science-fiction novel by prolific author A. Bertram Chandler, published as one-half of an Ace Double, along with "Empress of Outer Space", also by Chandler. Unlike many of Chandler's works, which deal with intergalactic travel at the edge of the galactic rim, this book (as the title suggests) is planetary science-fiction, although it is a sequel to another story, "The Coils of Time".
The front cover of the book says that the book was inspired by the real success of the Mariner missions, which established that Mars was a dead world. The book's introduction asks what happens to all the wealth of planetary romance when we deal with a lifeless Mars, far away from what H.G. Wells and Edgar Rice Burroughs imagined. The book addresses this, although in a roundabout way. Our protagonist, Christopher Wilkens, along with his wife, Vanessa, are on Venus. Events of "The Coils of Time" are discussed, where it turns out this Vanessa is a parallel universe version of his original wife, and that both of them have memories of alternative worlds. They speculate that these "memories" of different "time coils" are the source of fictional stories. After debating this for half the book's length, and also filling us in on some other prosaic details, they end up on Mars, and by accident they end up travelling to an alternate Mars, a combination of different fictional versions of Mars (some of which I was familiar with, and others of which I wasn't). The final half of the book takes a darker turn, as the protagonists end up on a Mars where human are used as slaves and food, and they have to fight against the grotesque Martian aliens and their human slaves. It is actually an exciting story, but it is compressed into the end of the book. As in many Ace Doubles, the editing was over-zealous, but in this case, the set-up for the book is too long, while the action is too short.
The summary of this book, as advertised, actually deals with an interesting issue: how real-life exploration of The Solar System in the 1960s made an entire genre of science-fiction obsolete. But the actual book didn't focus on that as much, and seemed to be just a chapter of a longer story, as A. Bertram Chandler, who wrote classical "naval" space operas, seems to have been developing a longer narrative, with this book just part of it. As always, Chandler gets points for developing ideas and worldbuilding, but as a reader, I felt like I was missing something.