(Easy answer; they're all good ones. ^_~)

You're coming in on characters who've already had a couple of books about them, but that's fine: Pratchett's lead characters are often fairly archetypical, so my advice is to just ignore anything that suggests you should know them already and roll with it.

The nice thing about Pratchett is that, while Maskerade assumes familiarity with a certain other famous story set in an opera house, the humour still works either way. I first read it before I had more than a vague understanding of the story it's referencing, and it was still excellent.

Signed,

hS!!!