This book falls into one of my favorite styles of spinoffs; it is the same story told from the perspective of another character. What this often does is fill in that characters background more than was included in the original, while still keeping to the original book's details. If done well, the book can seem like a seem-less extension of Austen's work.
Jane Fairfax is the story of Jane Fairfax from Emma. While Jane is an important character in Emma, we miss out on many details about her life, background, and more as Emma is put in the spotlight. This story moves that spotlight to Ms. Fairfax.
This book was definitely successful at retelling the story. I really felt like I was reading Emma for the first time as I made my way through the pages. Aiken doesn't create any impossible plot twists, but instead remains true to the story of Emma while enhancing it in small ways.
While Aiken doesn't write in the same style as Austen, I prefer that, as most modern writers would fail to match her language and wit present in the original. One of the only frustrations I had with the novel was the lack of passion between Churchill and Fairfax. While we learn more about why they may have been secretly engaged, Fairfax shows little towards him but some frustrations with his actions. Still, she waits for him. I'd love to have had this fleshed out a bit more.
Overall, a fantastic read. It ends a little quickly, but is a great companion to Emma, and allows us to learn to hate Emma a little bit more, while giving a little bit of sympathy to Fairfax.