My Lord John was Heyer's last novel, and is actually unfinished, with the manuscript ending right in the middle of a sentence. She had planned on writing the life story of Lord John, Duke of Bedford, son of King Henry IV and younger brother to King Henry V, but this book ends right before the death of his father. It covers his life from childhood through to 1413, when he was in his early twenties. Heyer did an enormous amount of research to write this book, and this becomes very obvious throughout the story. In addition to discussing the life of one man, we learn about the entire world during that time period, from details of the lives of the princes, to the struggles on the world stage.
I found this book to be incredibly difficult to read. For fans of Heyer's light romances, which are also excellent historical fiction, this book might be a bit daunting. It is very different from the others I've read by her. What made it most frustrating for me was the fact that I could not for the life of me keep all the names and titles straight, even with the help of the cast of characters at the front of the book and the family tree in the back. The problem comes from Heyer's use of not only the character's given names, but also their titles, which seem to be constantly changing, and even their nicknames, if the characters have them. Most historical fiction authors that I have read try to keep things a little bit more in order for their readers, as though they understand that this can be confusing. Heyer also uses language from the time, and she helpfully includes a glossary, so that her readers will not be further mystified about what is going on. Still, this use of language tended to add to my difficulty with the book.
Overall, the historical detail is incredible, and the reader can learn a tremendous amount about this time period from the book. It would have been good to read the entire thing, no matter how difficult I found it. However, I hesitate to recommend this one, simply because the constant name switching and use of language were for me very distracting.