The Catholic Book Club discussion

4. The first book is titled “Et in Arcadia ego,” the second “Brideshead Deserted” and the third “A Twitch upon the Thread.” What meaning do the titles convey?

Huh, my edition has only two "books," “Et in Arcadia ego,” and “A Twitch upon the Thread.”

“Et in Arcadia ego” is the title of a painting by Poussin (apparently a popular painter with English authors, I read Anthony Powell's A Dance to the Music of Time last year, which was inspired by a Poussin painting of the same title), meaning "even in Arcadia I am" it is traditionally understood that "Arcadia" refers generically to any utopia and "I" refers to death. Perhaps there are two "utopias" that could be being referred here: Oxford and Brideshead. Even though the war is over, WWI continues to cast a shadow over Oxford. Brideshead likewise seems to be suffering from the death of the marriage between Lord and Lady Marchmain.
(I got a lot of the background for the above from Wikipedia)

“A Twitch upon the Thread” is easier to understand, for Waugh explains it with a quote from Chesterton's Father Brown: "'I caught him' (the thief) 'with an unseen hook and an invisible line which is long enough to let him wander to the ends of the world and still to bring him back with a twitch upon the thread." There are in the end many such twitches - Sebastian yields to his perhaps vocation and wishes to serve as a lay brother in the remote wilderness, Lord Marchmain on his deathbed, Julia upon her father's death, and Charles in the chapel.