I remember reading the book about 15 years ago. It was full immersion VR in a medieval setting. I've looked at a number of potential matches such as, Killobyte or Heir Apparent and they don't ring a bell.
I remember one specific detail, the main character would go to the physical server location to play, because she (almost certain it was a she) thought that you would get the best connections if you went local. She's informed later that with modern connections this wasn't necessary. At one point she disconnects and looks around the room expecting to find some kind of culprit there in the room with her, playing local like she is. I think maybe people were starting to die off in real life, and she's trying to help figure it out?
The book was in English, fairly standard sci-fi length (Like Freedom's Landing comes to mind).
No recollection on the protagonists age, or the age appropriateness. I vaguely remember a battle scene in game, but don't remember anything beyond it being a medieval setting.