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).
Working Stock is the portion of the U.S. Government gold reserve which the Mint uses as the raw material for minting congressionally authorized coins and consists of bars, blanks, unsold coins and condemned coins.
The gold reserve held by the Department of the Treasury is partially offset by a liability for gold certificates issued to the Federal Reserve Banks at the statutory rate, which Treasury may redeem at any time.
U.S. Department of the Treasury. Fiscal Service, Book Value of U.S. Mint Held Treasury Gold Working Stock: All Locations: Coins, Blanks, Miscellaneous [MHTGWSALV], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/MHTGWSALV, May 26, 2019.