Roughing It contains Mark Twain’s famous review of the Book of Mormon (“chloroform in print”), with an oddly not-quite-right list of the contents:
The Mormon Bible consists of fifteen “books”–being the books of Jacob, Enos, Jarom, Omni, Mosiah, Zeniff, Alma, Helaman, Ether, Moroni, two “books” of Mormon, and three of Nephi.
This is mostly correct, except that there are actually four books of Nephi and no separate book of Zeniff. The record of Zeniff is contained within the book of Mosiah, comprising chapters 9-22 of that 29-chapter book. The omitted book, 4 Nephi, is tiny, consisting of a single chapter.
If some early Christian writer had given a similarly not-quite-right list of the Bible’s contents, we would take it as evidence that the Bible actually consisted of a different collection of books at that time, but in the case of the Book of Mormon there are so many surviving copies of the first and other early editions that we know that, while a few words have been changed from edition to edition, the list of books was never any different.