For a romance-focused take on the concept, 'The Two Lives of Lydia Bird' by Josie Silver is lovely. After her fiancé dies, Lydia discovers she can fall asleep and enter a dream-like reality where he is still alive. It's a poignant exploration of grief, letting go, and choosing between a perfect fantasy and a painful, real future.
The emotional journey is the core here, with the alternate reality serving as a metaphor for the paralysis of loss. It's a tearjerker for sure, but it feels authentic and ultimately hopeful. A great example of using a speculative element to amplify very human emotions.