This book has everything I love about regency romances: quirky side characters, a redeemed rake, and a social awkward MC who doesn't realize that the rake is obsessed with them. 
Using letters as a mean to show the story worked well. The problem is (and the main reason why this isn't a 5 stars) in the last 30% of the story, the MC spends most of his letter to his sister writing scenes that happened in his life. It makes sense to have the actual scenes for some of the most pivotal moment of the story, but it was weird to see these very long scenes told in a letter. I'm not sure how I would have done it differently, I just now that how it is now it's a bit awkward. Still, it's only a stylistic choice, which didn't impact my enjoyment of the book