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Humble Bundle - Sir Terry Pratchett's Discworld
(www.humblebundle.com)
Anything related to the fantasy genre
While there are myriad reading orders that have been devised over the years, my method with long extended series such as this one is simple: I consume them in the order in which they were produced/released.
I would slightly discourage that here. The Color of Magic is fine, but the Light Fantastic is a bit rough. Equal Rites is fine again, but doesn't really focus on the characters that make the Witches series good.