Personally, of the Narnia books I prefer The_Magician's_Nephew, because it has that wonderful vision of a dying world, and because the Empress castigates the English, "You treat your slaves poorly", to which they reply "But they aren't slaves". That exchange could make a middle-class child of the time think about the status of the working class.
But which ever book is your favorite, they're clearly in the short list of "possible best juvenile fantasy of the 1900s".
In There_Are_Doors, the death-after-intercourse requires that all women either bear twins or triplets or store semen for later pregnacies (as many insects do). Otherwise the species dies out. How you would evolve such a system is hard to figure out, and there's no implication that this is how mammals work in this world.
The evolutionary wierdness of the idea, coupled with the non-wierdness of it as one psychotic's belief about sexuality, seemed to me to be a hint that the "hero" is not in such a world--he's in our world, but he is insane.
-John Bishop