What about "Mr. Garfield?"

So I just finished reading Sarah Vowell's book Assassination Vacation
for the book club I go to sometimes at the Well Fed Head.
It's about the assassinations of Abraham Lincoln and James Garfield and William McKinley and all sorts of tangentially related minutiae. Interesting stuff really. I wouldn't have finished the book if that weren't the case. The first chapter, about Lincoln, was the longest and most detailed. The rest of the book went much faster.
The second chapter was about the assassintion of President Garfield. I was disappointed. As I read through the long chapter of Lincoln, I looked forward to reading something, anything, about the Johnny Cash song "Mr. Garfield."
But nothing.
I searched for Sarah Vowell's e-mail address to ask about this omission.
But again, nothing. Granted, I didn't search that hard, but with the internet, how hard is it supposed to be.
Perhaps if Sarah Vowell is the sort of author who googles herself, I'll get an answer. Maybe I should seek out her editor.