Rory reads this book by Stuart Gilbert while she waits for Lane outside the supermarket. First published in 1930, it’s a guide to James Joyce’s notoriously difficult 1922 novel Ulysses. Stuart Gilbert was the first French translator of James Joyce’s Ulysses, and through that became good friends with the author. His systematic analysis of the novel was written with James Joyce’s approval and input, so it’s seen as the official guide to Ulysses.
Rory must surely be reading Ulysses as well to need this companion book to help her understand it. It seems to be something she is reading for herself rather than for school, as we later learn they have been studying Henry James at Chilton.