With 33 (I think) of Terry Pratchett's Discworld novels published, I can't imagine how daunting it must be to attempt to introduce someone to the "series". In my day it was as simple as picking one of the six in existence (he was only up to Wyrd Sisters when I started) and carefully hiding it in my history book so Mr Osbourne wouldn't catch me reading about a certain wizzard.
Luckily, to the rescue is the Discworld Reading Order Guide which is maintained in several languages by Krzysztof Kietzman of Lspace.org. Phew. Making Money (the latest one) goes at the end of the Industrial Revolution line. Not sure that's particularly useful but I like things to be up to date.