I enjoyed this book. It's a well written tale about Isidore and Simeon, the Duke and Duchess of Cosway: "Married by proxy as a child, Lady Isidore has spent years fending off lecherous suitors while longing for her husband, Simeon Jermyn, the Duke of Cosway, to return." A light hearted story about what happens when a passionate, head-strong and beautiful woman collides with her calm, dispassionate (on the surface) and in-control husband. A lot of the story takes place on his country estate, which has been sadly neglected. There is lots of choice detail about Georgian plumbing! I didn't find it hugely emotionally intense but their developing relationship was well described and there is some good, snappy dialogue.
I can't help it though, I confess that in all the books so far I have been distracted by the parts featuring Jemma Beaumont and her husband Elijah. It is very tantalising - I want to read their story and because of the way it's been set up I'm guessing theirs will be one of the darkest books of the series. I'm also looking forward to finding out what happens to the Duke of Villiers.
I do like how Eloisa James writes and "When the Duke Returns" is definitely better than the last two in this series.