All I know at this moment: I know it was fantasy. There were characters named something similar to Demonicus (evil dragon sorcerer, quite powerful) and Heregio (evil fire sorcerer). In addition, I know the main character, a boy, lives in the modern world and works in a library or bookstore, or his parents own it, but he then travels to a fantasy world (I am not sure about the detail) and lives a life there? Eventually, falling in love with the daughter of the evil dragon sorcerer. I also remember that at one point in the book, that daughter comes with the main character to the real world, where he’s in an awkward position as his real girlfriend is also there.
I know that the tone was likely relatively dark? and the story itself quite intense, but I am not quite certain.

I also remember that somewhere in the middle, there was a clash between Heregio and the protagonist, but the protagonist survived, and Demonicus actually beat Heregio for betrayal.

It’s a book I read about 8 years ago in print from my local library in Dutch (likely a translation from another language)… I was still in primary school, but I read anything fantasy. I have been searching for this book for years, and it’s frustrating me I can’t find it.