The Writings of Albert Gallatin, ed. Henry Adams (Philadelphia: J.B. Lippincott, 1879). 3 vols. http://oll.libertyfund.org/titles/1951,
Vols. 1 and 2 contain Gallatin’s letters to people such as Jefferson, J.Q. Adams, and Henry Clay and Vol. 3 contains longer works such as speeches and reports on elections, the finances of the US, public lands, currency and banking, Oregon, the war with Mexico.
The text is in the public domain.