What was the first American novel to sell one million copies, who wrote it and when was it published?

"Uncle Tom's Cabin" written by Harriet Beecher Stowe in 1852.

Another one said to have been the first: Harold Bell Wright with his novel "The Shepherd of the Hills" written in 1907.

Or maybe this one: In His Steps by Charles M. Sheldon, published in 1896.

Uncle Tom's Cabin was the best-selling novel of the 19th century (and the second best-selling book of the century after the Bible) and is credited with helping to fuel the abolitionist cause in the 1850s. In the first year after it was published, 300,000 copies of the book were sold.

https://en.wikipedia.org/wiki/Uncle_Tom%27s_Cabin

Page 226 of Robert J. Scarry's "Millard Fillmore" states that "Uncle Tom's Cabin" was the first American novel to sell one million copies and that Millard Fillmore had two copies of this book.

http://www.shapell.org/manuscript/president-fillmore-uncle-toms-cabin-civil-war-recolonization-of-slave