There are so many! Always meant to compile a list, but I'm not so knowledgable.
Brian quotes Shakespeare once in 405, I think, debating self-justice with Ben and Michael at Babylon.
Then there's the Gorky (?) reference during Brian's first visit to the baths in 118, something with hell or such. Can't quite remember that one.
And I love the Poe (The Raven) reference in 409 when Michael comes to see Brian. Michael: You should be in bed, resting! Brian: I was - but someone kept knock-knock-knocking at my fucking door.