Several songs are mentioned in Boots & Backpacks, but I’m Gonna Be (500 Miles) by The Proclaimers appears multiple times. It is, therefore, the unofficial theme song.
How many times could you listen to that song before you got sick of it? I only ask because [spoiler alert!] in B&B, there’s a scene where Elizabeth does her best to make everyone sick of it. You’ll have to read the book to learn more! But here’s a hint, from the great comedian John Mulaney: