Witcher Signs is a book in The Witcher 3: Wild Hunt that can only be found as random loot in a few locations associated with magic users.

There are approximately 15 containers throughout the game that can have it, but due to a randomization bug they always select a specific book. The containers known to always pick this one are a second floor dresser and third floor bookshelf in Triss' ransacked house on Hierarch Square, and a skeleton where the gargoyles are during The Path of Warriors, and a bookshelf in the room with key to library during The Tower Outta Nowheres.

Journal entry

As a mule is neither ass nor horse though it has traits of both, so are witchers neither mages nor common men.

Witchers are able to cast simple spells they call Signs, drawing on the basics of telekinesis (Aard), pyrokinesis (Igni), hypnosis (Axii) and so on. They are not, however, capable of utilizing more complex spells - indeed, they treat real magic with reserve and distrust. Widespread in their ranks is an irrational, nigh onto superstitious fear of teleportation (let me remind you that deadly accidents during translocational travel occur only once per hundred instances of such travel!)

This aversion surely stems from the fact that witchers do not possess inborn magic talents, but instead gain them during their infamous Trial of the Grasses. It can thus be said that their casting spells is an affront to nature - as is, in fact, everything witchers do.