I'm sorry I couldn't give this book a higher rating. It's well researched and lovingly written. The authors obviously know and care a lot about Haydn and his music.
The problem is the book's format. We read about 200 pages worth of biography, which is pretty good, followed by 180 more pages describing Haydn's pieces one by one -- which, considering the sheer number of pieces he wrote, means that each piece gets a paragraph or so. There's no way around it: this is a terrible way to read about music. If you don't know the pieces, then reading single-paragraph descriptions of them isn't going to bring you any closer to knowing them. And if you do, then reading single-paragraph descriptions of them isn't really a very good use of your time.
I'm aware that the authors didn't invent this format, that for some reason there's a demand for books like this. I, however, am not one of the people who demands them. So, three stars for the biographical half of the book.