Pratchett's first two adult novels, "The Dark Side of the Sun" (1976) and "Strata" (1981), were both science-fiction, the latter taking place partly on a disc-shaped world. Subsequent to these, Pratchett mostly concentrated on his "Discworld" series and novels for children, with two exceptions: "Good Omens" (1990), a collaboration with Neil Gaiman (which was nominated for both Locus and World Fantasy Awards in 1991), a humorous story about the Apocalypse set on Earth, and "Nation" (2008), a book for young adults.