I love Eliot and have loved her since my first encounter with Daniel Deronda because it is a perfect and wonderful book and "only liking the Gwendolen parts" is on my list of "things that feel like they should be antisemitic even though they aren't really" alongside unboiled bagels.
This is not about that. I'm not sure how this book lands if you aren't deeply invested in Eliot already and what made her tick. But the story of her relationship with GH Lewes was always presented as either salacious or iconoclastic, but Carlisle's investigation here goes much beyond that. She sets out to treat Eliot like a character in an Eliot novel: what is the fullness of this person, her drives and dreams and flaws and situational complexity. Who does she long to be and how is she shaped by the people around her. The way Carlisle interweaves the books with the biography is either brilliant or contrived; I'm leaning towards the former, but not actually sure. But the final chapter where she gets to life post-Lewes is probably my favorite part. 
The book can't entirely stand up under what it's being asked to bear; to do that, it would need to be a novelization of Eliot's life, not an exploration through letters and philosophy and Carlisle seems to know that as well. It's not - quite - trying to be and the fact that Carlisle can't resist making it also a story about marriage and becoming in the way that Eliot's books are cannot be said to be the fault of the author when it is manifestly the influence of Eliot. I'm so glad I read it.