The Keeper of Lost Causes by Jussi Adler-Olsen

Copenhagen homicide detective Carl Morck survives a shootout, but his two partners aren't so lucky. One ends up dead and the other, Hardy, is paralyzed. The episode gives his superiors the chance to push him out of the department because of his difficult ways. They banish him to the basement of police headquarters to head Department Q, which will be responsible for revisiting old cases that were never solved. Carl is given an assistant, Assad, who helps him with his first case, the disappearance of politican, Merete Lynggaard. She went missing five years earlier while on a ship with her younger brother, Uffe. The reader follows Merete's imprisonment by unknown assailants and wonders if she'll still be alive to be rescued if Carl can trace her whereabouts. The Keeper of Lost Causes is the first book in the Department Q series and its strength lies in the partnership of Carl and his colleague, Assad, who it appears is not merely the Syrian refugee he claims to be.