In the games of The Witcher series Yennefer meets Geralt for curing of Dandelion who was harmed by the Jinn, but in the Netflix series we see a proper back story of Yennefer and her childhood and origin is shown. Which book of the series is taken as a reference to tell the backstory or have the writers of the series created that from their own?
Yennefer's backstory is aluded to in The Tower of Swallows:
Janka! Dear Janka!
Take this hunchbacked monstrosity from me! I don’t want to look at it!
She’s your daughter as much as she is mine.
Indeed? The children I have sired are normal.
How dare you… How dare you suggest…
It was in your elven family that there were witches. It was you that aborted your first pregnancy. It was because of that. You have tainted elven blood and a tainted womb, woman. That’s why you give birth to monsters.
It is an ill-fated child… Such was the will of the gods! She’s your daughter as much as she is mine! What was I to do? Smother her? Not tie the birth cord? What am I to do now? Take her to the forest and leave her? What do you want from me, by the Gods?
Get away, you freak.
How dare you! How dare you strike a child! Stop! Where are you going? Where? To her, are you? To her!
Yes, woman. I’m a man. I’m free to sate my lust where and when I want, as is my natural right. And I loathe you. You and the fruit of your degenerate womb. Don’t wait with supper. I won’t be back tonight.
Why are you weeping?
Why are you beating me and pushing me away? I was good, wasn’t I?
- Chapter 9, The Tower of The Swallow
And Geralt, after meeting Yennefer in The Last Wish and wondering what her former deformity must have been (most candidates for sorceresses having been 'unmarriagable' young women), discovers:
He knew what she used to be. What she remembered, what she couldn't forget, what she lived with. Who she really was before she had become a sorceress. Her cold, penetrating, angry and wise eyes were those of a hunchback.