Tomorrow and Tomorrow and Tomorrow by Gabrielle Zevin is a brilliant example of how you can use an unreliable narrator to your advantage in your writing.
This review contains spoilers!

There are many story building elements in this novel that I could have picked out, but the unreliable narration is so core to how the story unfolds that it would be silly not to explore it.
Tomorrow is told from the perspective of two friends, Sam and Sadie. Zevin uses this structure to pick over details that reader’s thought were facts. Any truth presented in Tomorrow is only truth relative to the narrating character. The role of the unreliable narrator is to challenge the reader’s perceptions of morality in the story’s characters.
Both narrating characters in this story are morally complicated, neither truly good nor truly evil: simply human. Zevin allows the reader to make moral judgements of the non-narrating character’s actions, and then challenges those judgements later on in the story.
What I think would have pushed this story to be even better is giving more depth to Marx’s character. Both Sadie and Sam hold Marx as a shining example of moral goodness. He can do no wrong. But we know that neither Sadie nor Sam can be trusted on their moral judgements. It would have been really interesting to see the outcome of Marx being as fallible as anyone else.