Pratchett's Discworld sounds as though the books are going to be silly, but he has really figured out how to balance ridiculous with deadly seriousness. I think the darkest novel was "Truth", but that's just my opinion.

Pratchett has Alzheimer's and can't write any more. I haven't read the last one or two he wrote, but I, along with everyone else who loves his work wishes there would be more.