Which American author wrote essays in the form of a letter?

The American author who wrote essays in the form of a letter is Henry David Thoreau.

His most famous work, Walden, is structured as a series of letters detailing his experiences living in the woods. While it's not technically a letter to a specific person, it reads like a personal reflection addressed to the reader.

Thoreau also wrote many other essays and journals in a similar epistolary style.