If the archer is also an apothecary, you might give him an ancient tome of elven lore that contains, coded within its elven verses, formulae for various useful compunds. As he progresses in level, assuming he continues to study the book (and maybe takes some levels in Decipher Script and/or History) he could find new formulae that add to a variety of things:
-sleep arrows like those possessed by the fey, ancient elven friends and allies,
-an acid that he can apply to his arrows, giving them acid damage without eating away the arrowheads,
-naturally occuring poisons with uniqe properties/abilities, (Slow as the spell, or an effect similar to Tasha's Hideous Laughter or, at higher levels, Otto's Irresistabile Dance) for which the book gives him some effective levels (specific to those poisons) of the Craft (poisonmaking) skill,
-more effective versions of existing PHB items (i.e. alchemist's fire, etc.)
-ways of replicating a few specific potions, depending on what you want to give the party and what they have already
Or whatever else seems appropriate for the player and the party...
Maybe have them meet the book's current owner, let the player help the owner based on some item or piece of info that the player already has, and then the player receives the book, a brief explanation of what might be hidden within its pages, and a clue to finding the first formula as a reward.