Just a warning! Once you start you will not stop. I finished the whole saga in a month. As well what I want to add is that the books really show Yen's relationship with Geralt and even more with Ciri. CDproject Red didn't have enough time to really flesh out Yen's character in this game such is why it would seem to a lot of people that she acts unpleasantly. Ciri actually has more of a connection with Yen than Geralt. After you read the books, I'm 100% sure you will change your views on Yen, maybe even prefer Yen than Triss for various reasons. But I'm not gonna jump into spoiler territory! Pick up the damn books and stop gaming!
I came across a sort of executive summary on the web, and the few paragraphs there on the three of them gave me enough background to have a better understanding of things and feel better about romancing Yen (and searching for Ciri).
Perhaps if there were a little more in the game, there would not be such a strong support for Triss in certain quarters. From my perspective OOC, it's as if I've been told that Yen was my lover, but I still have no memory of that so it has no meaning. Geralt of course has regained his memory, so I felt I needed some context for that. I've got it now.
I might read the books some time, maybe in the downtime this winter. As I said earlier, I used to read a real lot. I have hundreds of books boxed up in the attic. I just sort of got bored of reading eventually and found it hard to find anything 'new' that satisfied me. Perhaps it's time to try again...