Hell yeah, Pratchett is great, and as for which ones to read, I suppose it depends on what series sounds most interesting. His best works involve the City Watch (Guards! Guards!, Night Watch, Feet of Clay, with the best so far being Thud!), then my personal favorite Rincewind and his follies (The Colour of Magic, The Light Fantastic, Sourcery, Eric, Interesting Times, and The Last Continent). His first book, The Colour of Magic, is pretty weak, but he definitely improves quickly after that. Also the series involving the witches is pretty good (Witches Abroad, Lords and Ladies, a few others) and several stand-alone works that don't feature returning characters (Small Gods was really funny, for instance). Otherwise, check out Good Omens, which he wrote in cooperation with Neil Gaiman. One reviewer said that it read like the Book of Revelations if it had been written by Monty Python. Good shit.