I've finished the Watch books, and I want to start reading one of the other series. I was about to purchase the whole witch series, when suddenly I wasn't quite sure if that was the best decision. I've been thinking about the Death series as well. I've read Soul Music, and I liked it a lot. I've seen the Hogfather TV-series, and didn't like it quite as much. As for the Rincewind series, I've read Color of Magic, Light Fantastic, Interesting Times and Sourcery. I must honestly say that the Rincewind series didn't appeal to me as much as the Watch series did. So, with that in mind, which series would you recommend I purchase, and why? (please try to leave out any big spoilers)
Personally I just read each book as they came out. The Watch Series, Witch Series, Death Series etc aren't really a series as such, they were just grouped that way as an after-thought by the publishers so people can read all the books with the Witches as a focus or all the books with Death as a focus. They were never written as a series. If you aren't sure which series to read I'd say don't read any of them, just buy the individual books from start to finish, obviously missing out the ones you read already.
And for sure don't pass up the Y/A books just because of that label, they are just as rich and wondrous as the more adult Discworld books. I think the witch books are probably more like the watch books then the Rincewind "series" too. The witches have a lot in common with the watch. But as a rule all the books are worth having and reading more then just once.