Department Q (Jussi Adler-Olsen) is a book series that I recommend as summer reading. First of all, it has an interesting title, at least in my opinion. Department Q. In these books, Department Q (Carl Mørck and his team) is solving cold cases. I started reading book number 5, The Marco Effect, yesterday and at least so far I have liked these books. There has also been made movies based on these books but I haven't watched any of those yet.

https://en.wikipedia.org/wiki/Department_Q
https://jussiadlerolsen.com/