Book value is determined such that the Department of the Treasury records U.S. Government owned gold reserve at the values stated in 31 USC § 5116-5117 (statutory rate).
Deep Storage is the portion of the U.S. Government-owned gold bullion reserve which the Mint secures in sealed vaults that are examined annually by the Treasury Department's Office of the Inspector General and consists primarily of gold bars.
U.S. Department of the Treasury. Fiscal Service, Book Value of U.S. Mint Held Gold Deep Storage: Fort Knox, KY [FKKYGBV], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/FKKYGBV, May 22, 2019.