Solitaire is a novel written by Alice Oseman, which was published on July 31, 2014 in the UK, by HarperCollins and it was also published on January 3, 2023 in the US, by Scholastic Inc. It is Alice's first novel.
It is set in the same universe as Radio Silence, Heartstopper, I Was Born For This, and Loveless, right after This Winter. Which means that there is a slight overlap between Heartstopper and Solitaire. Heartstopper Volume Four briefly covers Solitaire without getting into the main plot.