I only read The Last Wish, Sword of Destiny and Blood of Elves.So far, alchemy and Signs are practically nonexistent. I think Sapkwoski use them foremost to add credibility to his world. You see, witchers are supposed to be a mysterious, 300 years old, order/caste, specialized to fight monster. So it would be weird that they don't come up after all this time with a whole body of knowledge and practices (martial art, alchemy, sign, mutation and co). But Sapkwoski don't have to go into details about such topics. Brief mentions here and there are enough to create the illusion of a complex framework, and so create the right amount of credibility for his witchers.So, for alchemy :In blood of Elves, we learn a bit about white gull and black gull.White gull seem to be a calming, relaxing concoction. Witchers drink it during "long winter evening" to have good sleep. Black gull is a bit the same but way stronger, to the point of being hallucinogen and toxic.Note that Triss took a sip of a white gull, but couldn't recognize any component, so it contains some of the secret witcher herb and plants, which even sorcerer don't know. Black Gull seem also to match the description of the potion Geralt took in Sword of Destiny, to slow down an infection due to a septic bite -> As a side-effect, the potion threw him straight away in a weird, near coma-state, full of hallucination.Godkingofdivineroad you asked a question about witcher Sign in another thread.It's the same pattern. We have only very vague and scarce evidences.For example, in the short story "A Grain of Truth", during the battle with the Bruxa, there is a very quick reference to the Sign of Quen. We only know that Geralt cast it, and then the bruxa "broke it" with a powerful shock wave. So we can maybe deduce that Quen is a sort of shield, but that's not sure.Axii seems to be the Sign that Geralt use, to hypnotize/stunt his horse. (in "A Grain of Truth", again)In Sword of Destiny, Geralt use what seem to be Ignii, for burning a rope.In Blood of the Elves, Yennefer talk a bit about Aard, and how it's a degenerate version of a true (and way more powerful) sorcerer spell. And that's it.So, even with four books left to read, I am pretty sure that Sapkwoski never give a full exposition of what alchemy and sign really are. It's not his aim.It's look like a blessing and a curse for CDProjekt, because they have very few material to work on, but on the other hand it give them a wider latitude to build a good gameplay system.