There are two things to discuss about this book. The first, is the Venusian landscape and the second is the religious elements to the book. The landscape is a wonderful piece of surrealistic planet design. It's far superior to Lewis very good Mars and is set in an ocean with very few bits of fixed land and lovely exotic landscapes. It is really excellent and beautiful.
The second is the religious elements. It is easy to describe this as a retelling or analogy of the Fall story. That's not quite true, as the story deliberately parallels the Fall story only in parts and diverges from it in other parts. So an alternate story similar to but in the end different is presented as part of Lewis' rather complex religious beliefs. He actually takes seriously the idea, that the Christian story is unique with a creator who has created multiple worlds with intelligent species and tries to reconcile that while articulating Christian Theology and exploring the nature of good and evil within that. That's all very bold and quite a task for a novel, but it is only partially successful. I'm not sure that I find his religious positions all that compelling, and his value system does not match with my own. The book, however, has a great Satan in it. The main character Ransom, who is an idealized ordinary Christian is problematic and his Eve is no great shakes with misogynists attitudes more on display here than elsewhere.
Overall the book is remarkably ambitious. It is more a work of theology than perhaps any other novel I have read. It's landscapes and Satan are great, but it sort of fails to work in the end. I have mixed feelings about it.