From the liner notes for SONGBOOK:
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16. A LESSON IN LOVE
I was inspired by reading a natty little book on an airplane one day, a biography of Phineas T. Barnum. The song is sort of loosely arranged around his relationship with Jenny Lind, a Swedish singer who toured America in the late 1800s. I reach a long way back in these tunes, back into my personal memories, and sometimes back into history.
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There's a discussion of this on the newsgroup back on September 22 1997. Google "A Lesson in Love" +Lightfoot in the Google Groups box and it's the third thread down.
For more on Jenny Lind see
http://www.oldekinstongazette.com/jenylind.htm