“Axel von Fersen” by Herman Lindqvist
This is a biography of Axel von Fersen and contains information about his entire life. I found it very interesting although not extremely entertainingly written. There’s lots to learn about Fersen himself, but also about other people around him, such as Gustav III, and interesting anecdotes about 18th century court life. There are many excerpts from Fersen’s own diary for example.
The Fersen and Marie Antoinette question is controversial to some, but although it’s obvious that the author personally believes their relationship was sexual at least once, the book doesn’t state anything as absolutely true or false.
It’s definitely worth a read, both if you’re interested in Fersen or just 18th century life in general. The ending is really sad and horrible of course, because of his gruesome death. The book is available in Swedish and Finnish.