Marcus Bach describes the Spaulding theory.
Marcus Bach, The Mormon (Salt Lake City: Deseret Book, 1951), 15
"What do you think about the miracle?" I asked.
"That's what it was!" proclaimed the boy. "A miracle!"
"What makes you so sure?"
"If it isn't true," said the boy confidently, "where did the Book of Mormon come from?"
"It could have been made up," I told him.
"By whom?"
"By anyone. By Joseph Smith himself."
"That would have been even more of a miracle. He had no education."
"There's the story of the Spaulding manuscript," I suggested. "It's said that a Presbyterian minister by the name of Solomon Spaulding wrote a story very much like the Book of Mormon. The manuscript fell into the hands of a Baptist named Sidney Rigdon, who in turn was accused of giving it to Joseph Smith. Now that might be where the Book of Mormon came from."
Instantly the young man spoke up, "That story has been conclusively refuted by the Church."
"You are a member?" I asked.
"We both are."
"Oh, I see. Well, I... well, anyway, that's the Spaulding story."