What's the minimum background reading for Pratchett's Going Postal and Making Money ?

I have a friend who I think would greatly enjoy those two books (possibly the whole series, but definitely those two), but who does not generally read fantasy. I’m thinking Guards! Guards! to establish the Night Watch characters and maybe Vetinari, but I’m not sure which book would most concisely deliver the feel of the setting – the Rincewind stories don’t seem to sit in the same ‘verse, alas.