Not only was Joseph Smith surprised by the content in the Book of Mormon but he also seemed to not know in advance how the book was structured. This would be quite odd if Joseph had planned the book beforehand and was simply reciting it from memory.
Joseph indicated to his scribe that a section had finished by dictating the word “chapter”. This word would be immediately included in the manuscript but the actual chapter numbers would be inserted later. It seems Joseph did not know in advance whether he was starting to dictate a new book or another chapter within the same book.
Some chapter numbers were added but later crossed out and renumbered within the context of a new book. In fact, Oliver Cowdery changed 2 Nephi 1 twice after realizing the chapter was a new book and then again after realizing there were two books of Nephi.
If Joseph was reciting the Book of Mormon from memory, why didn’t he indicate (or accidentally give away) when each new book started? Why would he add an unnecessary complication to the translation process?