I have a few favorites that capture this trope beautifully. 'Norwegian Wood' by Haruki Murakami is a hauntingly poetic tale of love, loss, and longing. The protagonist's unreciprocated feelings are woven into the fabric of the narrative, making it a deeply moving read. Another standout is 'The Time Traveler's Wife' by Audrey Niffenegger, where the non-linear timeline amplifies the agony of love that can't always be realized in the moment.
For a more contemporary take, 'One Day' by David Nicholls explores decades of friendship and missed opportunities, leaving readers with a lump in their throats. 'The Song of Achilles' by Madeline Miller also fits this trope, with Patroclus's devotion to Achilles being both heartwarming and heartbreaking. These novels don't just skim the surface of unrequited love; they dive deep into the emotional complexities, making them unforgettable.