I would start with Brideshead Revisited. Mr. Waugh was a convert to Catholicism and this was written post-conversion. He witnessed a death bed conversion which became the source for a portion of the book.
The book deals with the working of grace and it beautifully written, almost geometrical in form. The sentences, the paragraphs, the way an idea touched upon echoes and re-echoes, Waugh was a genius.
You might also enjoy a letter or two concerning Brideshead Revisited from the book of his correspondence- I recall a note, I believe in response to Nancy Mitford who suggested that he might be on the side of Lady Marchmain, responding that he thought God might side with her. Mitford’s semi-autobiographical novel “Love in a Cold Climate” will introduce you to a friend of Waugh’s who did not share his faith.
Rereadings of Brideshead will not fail to delight.
You might also read Chestertons’ The Innocence of Father Brown (detective fiction) to thoroughly enjoy the Twitch upon the thread quote which sets off the second half of the novel. The quote surfaces several times in the story as well as being used as a subtitle and has great significance, plus, it’s a good excuse to read some Chesterton fiction.
Enjoy!!
And then you are ready for the 10 + hour BBC television series (not to be confused with the loathsome 3 hour movie attempt). The long form was directed by someone unfamiliar with the book, so he had to read it each night and shoot it each day. Subsequently, it’s very faithful to Waugh’s writing.