The complete book is here – 1984 edition. Elevations are in Nassau County Department of Public Works datum which is the same as NGVD 1929 – mean sea level at Sandy Hook, New Jersey.
Hint to using this document: The Introduction has brief instructions at the end of that section. What’s missing is the key point that the bench mark entries are organized by plate number and that the plate number is listed on the far right of the entry. In the original publication this was obvious because the bench mark pages corresponding to each plate immediately followed that plate. Also note that Nassau County calls a “plate” a “board” – I just don’t like that terminology.
Map plates arranged geospatially: