The Immortal Dinner is certainly a good read. I hadn't realized it had gone out of print!
I had a nice talk about the book with the librarian at the Athenaeum of Ohio/Mt St Mary's seminary where Haydon's giant painting currently lives. (It's had some adventures through the years, and appears to be loved and protected where it is now.)
The digressions were among the things I liked best about the book, another favorite bit was the peek into the social habits of the era. (In other words, cheap time travel!