The Inkling Charles Williams died in 1945, and came out-of-copyright last year - immediately, The Charles Williams Collection, including all his novels, has been issued as an ebook on Kindle by Karpathos publishers at a give-away price of 82p.
If you have been intending to read some CW but not yet gotten round to it, then now is the time. Most people find either The Place of the Lion, Descent into Hell or All Hallows Eve to be the best of these novels.
Place of the Lion was one of the key books that sparked-off CS Lewis's serious prose-writing with Lewi's Space Trilogy and Tolkien's Lost Road/ Notion Club Papers and the legends of Numenor - which eventually got into Lord of the Rings and The Silmarillion. General enuthsiasm for Place of the Lion among the Inklings (it was Nevill Coghill who 'discovered' it in 1936) also sparked the friendship leading onto Inklings membership three years later and for the rest of CW's life.
I found it a very potent book which worked on me after I had finished it, so I have returned and re-read it several times over three decades - each time liking it more. The special philosophical interest comes from the implications of Platonic Archetypes; but in general PotL is a compelling description of the invasion of mundane life by the super-natural - the profound realities.
Descent into Hell is powerful and memorable, with a compelling inner logic. The philosophical interest is about the nature of Time: the novel convincingly displays the implications of divine time as described in Boetheius's Consolations of Philosophy (which is the understanding of nearly all mainstream Classical Christian theology - God outside of Time; observing past, present and future simultaneously - but not typically of Mormon theology).
All Hallows Eve is the most novelistic and flowing - presumably because it was written with the benefit of the Inklings criticisms and advice. Jack and Warnie Lewis felt it demonstrated an astonishing understanding of life after death.
(For me the elements of sadism in AHE are too prominent - and this is an undercurrent in all William's books except for Place of the Lion. It is a thing to which I am particularly sensitive and averse in fiction - a fact for which my family could vouch, when it comes to watching TV and movies. For example, I initially could not get-into the first book of Harry Potter - which I now love - beause I felt a careless cruelty about the way it described Harry being treated by the Dursleys!)
The other Charles Williams novels are interesting in patches, rather than enjoyable, to me - and I have never succeeded in reading any of them straight-through. The exception, and the most distinctive of them, is The Greater Trumps; which is half very good and half botched.
I don't regard Williams as a great or even canonical novelist - but three significant successes out of seven is not bad - and these include books which have meant a great deal to some of the great authorities of English Literature; notably including the prime arbiter of quality in the mid 20th century, TS Eliot. So, and especially at this price! - they are well worth a try.