I thought 'Five Feet Apart' wrapped up in a way that felt true to its characters, even if it wasn't easy. Stella gets her lung transplant, which is the whole goal she's been fighting for from the start. Will, knowing he can't stay with her because of his B. cepacia infection risking her new lungs, leaves without saying goodbye. He leaves her a painting of her in Paris, fulfilling that dream for her in his own way. The last scene is Stella walking through Paris, living the life she wanted, but you're left wondering if she ever hears from Will again.
It's an open-ended, bittersweet kind of ending. She gets a future, but it comes at the cost of their relationship. I remember finishing it and just sitting there for a minute, feeling that mix of hope and heartache. No, there isn't a sequel novel. The story is complete as it is. Rachael Lippincott, the author, hasn't written one, and the film adaptation ends the same way. There's some extra content like deleted scenes and the 'How It Ends' short film from the DVD, but those just expand on the existing ending, they don't continue the story.