I recently read both Strata and The Dark Side of the Sun by Terry Pratchett.
I am a big fan of Terry Pratchett; I read most of his books in my late teens including all the Discworld series and they left a big impression on me. Both Strata and The Dark Side of the Sun predate the discworld. Strata is the most interesting to me because although it’s ostensibly a sci-fi novel; it contains the theme of the Discworld. It almost seems like the Discworld is trying to burst out of the pages and become the main story :)
Although I think they’re interesting sci-fi by themselves, they are arguably not as well written as many of his later novels, so I wouldn’t necessarily recommend them to someone who hasn’t already read a lot of the Disc.