Charles Williams's War in Heaven starts better than it ends. It was more fun when the chief villain was a sinister publisher of occult books ("a particular kind of occult, the standard work on the Black Mass and that sort of thing", to quote another character) with private designs on the Holy Grail, aided by a cynical anthropology scholar who doesn't believe in God or magic but regards the publisher as an entertaining religious maniac.
but then Williams gives the publisher some associates, and they're all shifty stereotypical foreigners with more powerful sorceries at their beck, and thus War in Heaven resolves into a battle between true-hearted English Christians vs. diabolical alien magicks and becomes far less intriguing. (sighs) Honestly I feel like Williams lost his nerve halfway through and took his story into safer, pulpier territory.
Also Prester John turns up, as the deus ex machina and protector of the Grail. make of that what you will. https://en.wikipedia.org/wiki/Prester_John
~Chara