Nerd stuff: I haven't read the books, never will--but I did hear in one of the youtube videos, one of the CDPR devs, mentioned that the author of the Witcher does not consider any of these games as cannon, because well, Geralt is dead at the end of the books, and that is how the story ends.
Looking at the interesting video about the very early promo version of Witcher 1, the first version was a create-your-own witcher design, and Geralt was to only appear briefly (possibly as part of that back story). FWIW, I'm glad they decided to stick with a single character, pre-designed, with an established personality and history. It makes for much better story and RPG elements overall. Something that you can't really get in TES games.
I guess Yennefer is a big part of the books, and since Geralt is amnesiac in the first 2 games (I've always hated this cliche--but I mean, he is now not-dead, so I'll let it go

), she really wasn't part of the non-cannon game world until now. I'm guessing that with Geralt's memory being back now, this is CDPR really trying to put the story back into the story of the novels, and Yennefer is a nod to the earliest of Witcher fans? I dunno.