Elsewhere on this site, Kathy Hutchins briefly mentioned the novels of twentieth century British writer Charles Williams. I agree with Kathy's implicit assessment that Williams's novels are excellent.
I think that their decidedly lower popularity compared with the major works of Williams's friends Tolkien and Lewis is explained by the great difficulty most readers encounter in understanding precisely what is going on in Williams's novels.
One problem is that the books do not lie in a single identifiable genre. They are part horror, part fantasy, part mystery, part action-adventure, and all simultaneously. The events are those of romance, but the texture is of a realistic novel. One's expectations continually prove wrong.
The greater difficulty, however, surely lies in the nature of the world Williams depicts. It is exactly like our own, except for one thing, and this thing makes it so unlike our own as to be continually puzzling.
In these "spiritual thrillers," ideas and concepts from the spiritual realm manifest themselves in the natural world, though they are not immediately identified as doing so. If that seems a rather difficult explanation to grasp, it is because the concept itself is something that is best experienced rather than summarized.
However, once one overcomes the surface strangeness of Williams's narratives, they are quite compelling.
I would suggest starting with the most conventional of his books, War in Heaven (1930), his first novel. It tells the story of the Holy Grail having been found in a country church, and recounts the efforts of two groups to gain control over it. If this sounds rather like Tolkien's later Lord of the Rings trilogy, one can only note that great minds think alike, especially when they are friends and read each other's books. (Obviously the influence in this case would have been from Williams to Tolkien.)
Most of Williams's novels can be obtained online through used-book services, and some are available as etexts. Project Gutenberg Australia offers a page where some books no longer in copyright in Australia are available online. (These books are still in copyright in the United States and many other nations.)
An excellent introduction to Williams's fiction is available online here.
I highly recommend Charles Williams's challenging and rewarding novels. Careful reading of them will fully repay the effort expended.