Inside a crate on level 1, you find
A book describing in great detail events spanning multiple countries with hundreds of key characters over the course of approximately one century, it is uncannily elaborate. All dates, names and places are only referenced by numerical code, with note "See Book of References."
Attached is a Book of References, in this book there is a complete list of all important dates, personnel and locations of 20th century. Each with corresponding numerical code.
None of codes in one book matches the other book.