The colour of magic is, iirc, the first one Pratchett wrote, but I think it's generally regarded that he really got into his stride after a few books, so there's a case to be made for jumping in a bit later on in terms of the historical order. If you look online, you'll find some family tree of which books involve characters from prior books (e.g.
http://bestfantasybooks.com/forums/t...in-order.2244/) so if you want to really follow characters through a strict order, that gives you the chief clusters (the guards books, the witches books, the books focused on death...)
My experience of jumping about and ignoring the order, after reading the first ~10+ books way back in the day, is that it doesn't really matter - most of them stand alone in terms of plot, and the characters are strong enough that even when you read the back story out of order, it's still ok, like a BS Johnson style reordering.
Last edited by kokiri; 01-22-2016 at 03:53 AM.