CS Lewis’ Space Trilogy, starting off with the first book, “Out of the Silent Planet”.
It has a lot of theology in it. Some of it is fairly obvious. Other elements you probably wouldn’t see immediately because they are more subtle and may require deeper analysis and cross reading wth the Bible and also a touch of Plato. It’s also a thrilling story in its own right rather than an in your face lesson.
The second book in the series, “Perelandra” then moves away from broad theological concepts and homes in onto the specifics of temptation and sin, and especially the fall of man, again without ever getting too teachery or negative, despite the seriousness of the topic.
The third book, “That Hideous Strength”, takes place back home on the earth in a sort of dystopian academic setup with scientists running out of control. It was supposeldy a major inspiration to George Owell’s 1984, but in contrst to Orewll it has a strong Christian message.
Lots of interesting strating points for discussions and further stidies throughout.