Man, I keep circling back to old-school werewolf stuff when this comes up, but the classics like 'The Shining' or 'I, Robot'? Not the right alley. For that deep, unsettling 'who am I now?' churn, you want the ones where the change isn't just a costume swap. You want the psychological fallout. Clive Barker's 'The Thief of Always' does something clever with it, more metaphorical, but for a raw look at a man losing his humanity piece by piece, you can't beat 'The Metamorphosis of Prime Intellect'. It's not furry, it's a tech horror where the protagonist's consciousness gets uploaded and then... well, he becomes something else entirely. The old self is just data in a new system, and the book forces you to sit in that existential dread with him.
Then there's the flip side in romance, where the transformation is tied to belonging. Take the 'Alpha and Omega' series by Patricia Briggs. Charles Cornick isn't just a werewolf; he’s the enforcer, his identity is wrapped in that violence and control. Watching him navigate a bond that fundamentally alters how he sees his own role? That’s the grip. The change here is internal, a reshuffling of his entire sense of purpose, not just growing claws. For a pure fantasy take, Martha Wells' 'The Books of the Raksura' has a protagonist who discovers he's not human at all but a shapeshifting Raksuran consort. His whole journey is unlearning a human identity he built for survival and accepting a predatory, alien social structure as his true nature. The tension is constant because the transformation is permanent and social.