1) We give it to Emma, and she learns alchemy, focusing on potions for offensive or defensive purposes. Of course, this depends on whether Lock agrees to this option.
2) We give the book to Aldric, and he learns restoration. You can never have too many resurrections.
Maybe I'm missing some good options. I'd be happy to hear them. I suggest giving one potion to Ilse, hoping for the same extract. In that case, if Lock decides to develop alchemy, he can take the offensive potion. I also suggest giving one to Relhiras so that he can finally acquire the skill to wear armor.