In Moses 6:51-68 Enoch quotes from the book of Adam. This echoes 2 Enoch 33:11-12 which reads:
For I will give you an intercessor, Enoch; Michael, on account of your handwritings and the handwritings of your fathers–Adam and Seth. They will not be destroyed until the final age. For I have commanded my angels to guard them and to command the things of time to preserve the handwritings of your fathers so that they might not perish in the impending flood.
How would Joseph Smith know that Adam kept a book?