Here are a few recommendations for you:
"1984" - George Orwell - the classic dystopia, it's a great book in itself and a pretty pessimistic look into the then future.
"Brave new world" - Aldous Huxley - the second classic dystopia and the superior of the two if you ask me. When you read it, if you strip away some of the sci-fi elements and look into the core of the ideas, you'll see some interesting similarities with our own world at the moment.
"We" - Yevgeny Zamiyatin - only if you've enjoyed the above two.
"Metro 2033" - Dmitri Glukhovsky - a much better story than the game honestly, with great world building and exposition, but feeling even more depressing than the game did.
"The lies of Locke Lamora" - Scott Lynch - a currently running series named "The gentelman bastards", the first book you can read on its own and love it as it is, without having to keep going with the series if you don't want. Great prose, great characters, humor and a very twisty, dark story.
"The prince of thorns" - Mark Lawrence - great fantasy with a twist and a rather edgy, dark character...who happens to be a teenager. The more you hate him, the more you'll show your age.
"Flowers of Algernon" - Daniel Keyes - sci-fi about enhancing intelligence and how that impacts a man's life. One of my all time favorite books that I've ever read.
"The Ketty Jay" series - Chris Wooding - they encompass, in order: "Retribution Falls", "The black lung captain", "The iron jackal" and "The ace of skulls". I know you don't want a series, but this is a must read if you're just looking for light and fast entertainment.
More if you need.
