The Up-to-Date Bartender's Guide by Harry Montague (1913, p.49)

1 ozsassafras — sassafras rootbase
5 gallonswater dilution
0.5 gallonwater — Added while hot after boiling.dilution
1 teaspoonfulallspice — ground allspiceother
1 tablespoonfulginger essence — essence gingerother
1 ozhops other
1 tablespoonfulspruce essence — essence spruceother
1 pintyeast other
  1. Boil the sassafras root, hops, and water for 20 minutes, then strain.
  2. Add the extra hot water, ginger essence, spruce essence, allspice, and yeast when cool, let stand 24 hours, then draw off and bottle.

The source continues with a separate wahoo beer formula; only the spruce beer portion is curated here.