I too read some of the books, and was left scratching my head over the pair. I think "love at first sight" is the only explanation. The heart wants what it wants.
The one (and maybe only) scene in the game that provides some insight to Yennefer is the scene with the priestesses right after they discover their sacred grove has been spoiled. It may depend on dialogue choices, but she takes the fall for the whole thing (as she should). She cares deeply for Geralt, but rarely demonstrates it.
In the game world, it often feels like magic users are lumped with Witchers in the level of disdain the general population shows. Geralt usually takes it in stride, but Yenn tends to fight back in both word and attitude.
AvallacHRoche: I hate book whatever it is
it's clear - some may love books
Without books, there would be few games, and less knowledge in general.