So I have this friend who insists on doing "pre-reading" before he starts Discworld, despite pretty much everyone's suggestions otherwise. He's already chewed through most of Conan the Barbarian and Fritz Leiber's Swords books, and plans to spend the rest of the year on Lovecraft and Pern. Anything else that's a really big source of Sir Terry's lampoons?
oh gosh this is a huge question. I honestly feel like just discovering all the references and homages in the series would take a lifetime, but off the top of my head some of the more major ones for specific books were: Phantom of the Opera (Maskerade is more or less a direct parody), the Book of War (for Interesting Times, in which the disc equivalent of it is referenced), Les Misérables (for Night Watch, if there is one I could insist on it’s this one), Macbeth (and tbh most of Shakespeare’s classic plays, for Wyrd Sisters), maybe some Anne Rice (for Carpe Jugulum)…
I’d also suggest that before every book pertaining to some specific aspect of our society, your friend just spends a fun evening on wikipedia reading up or refreshing his memory on stuff like Egyptian gods (for Pyramids) and Greek mythology, and the major Greek and Chinese philosophers (for Small Gods, Thief of Time and some others), the history of old hollywood - its major actors and first movies, rock music, classic fairytales, the invention of printing, the functionning of post, trains, etc.