The Library of Congress has an extremely rare imprint of "L'Armonica: Lettera di Beniamino Franklin al padre Giambatista Beccaria…" (Turin: Stamperia Reale, 1769). It's one of only four copies known to exist. The pamphlet transcribes Franklin's letters to Father Beccaria, Italian philosopher and scientist, in which he describes the armonica in detail.

It's also illustrated with a diagram of the armonica, which you can see
here; scroll to the bottom and click on the image for a larger view.