After checking my records it appears that Spruce Beer was in a different manuscript than I remembered. Instead of a late 18th century book it is in an early 19th century book kept by a woman living near Bristol who collected many foreign recipes. Her recipe includes treacle and uses 20 gallons of water. This makes it a smaller quantity than most of her brewing recipes that call for enough to fill a large cask. Although the recipe is not dated, it may have been copied from an earlier manuscript book.
The first print version of a Spruce Beer recipe in an English cookbook that I have found thus far is from a dictionary of cookery titled, The Cook and Housekeeper’s Complete and Universal Dictionary, by Mrs. Mary Eaton (1823). Under the entry “American Spruce” the reader is directed to harvest “the young shoots and tops of the pine or fir trees” in the spring, or their cones in the autumn, and to boil this with water to extract a syrup the consistency of honey. This can then be used to brew Spruce Beer.
This is not the earliest reference to Spruce Beer in English sources, however. Captain Cook used it to fight scurvy, and a variety of mid-18th century sources included either directions for growing American trees that sap could be collected from or references to people brewing and drinking the beer.