Happy Birthday, Turk! No Exit 18 Years - Jakob Arjouni

A Turkish worker, Ahmed Hamul, is stabbed to death in Frankfurt's red-light district—certainly no reason for the local police to work overtime. Kemal Kayankaya, however has a different attitude. He is 26, born in Turkey, raised in Germany and now working as a Private Investigator. He has a German passport but has first hand experience of resentment against foreigners and now Hamul's wife, Ilter, has hired Kayankaya to find out who murdered her husband.
In the 3 days it takes him to wrap up the case, he has time to identify Ilter's sister as a heroin addict, track down Ahmed's girlfriend (a prostitute), link his father-in-law's fatal accident three years earlier to an ingenious police cover-up, and still survive beatings, gas attacks, and a close encounter with a Fiat.