
Some new hints:
The author is most famous for a (non-SF) book about a young knight who has to deliver a message to the king about an upcoming war, which has recently been turned into a (Belgian) movie.
There have also been plans to make a movie of the book I'm looking for, but there is a problem: while the two towers are very mysterious in the book (in the rural society of the parallel world, there is nothing like them), showing them in a movie would immediately take away their mystique because they are the most ordinary 20th century high-density residential apartment buildings.