As usual, I read this book in the original German, for one of my college courses, so I can't speak for the translation. The style in German is pretty simple, so most of the book should translate well. However, there is a lot of dialogue in some German dialects, and I don't know how that translates into English.
The story is pretty simple. Kemal Kayankaya is ethnically Turkish, but he was raised by a white German couple, so he doesn't feel like he belongs in the Turkish minority. He still looks Turkish, though, so he has problems fitting into German society. A Turkish woman, Ilter Hamul, comes to Kayankaya for help because he's Turkish.
Ilter's husband, Ahmed Hamul, was murdered, and the police aren't investigating. Kayankaya starts working on the case on his birthday (that's where the title comes from) and solves it within three days. The case becomes much more complicated than a simple murder, and involves drugs and corruption, and a very brief look at some of the troubles Turks face as a minority in Germany. That's kind of a bonus, because this is basically a simple, hardboiled detective novel.
Kayankaya is a good example of a hardboiled detective, so if you like hardboiled detectives, you'll like this book very much. Even if you don't, you'll probably enjoy it anyway, because it's well written. I usually don't like mysteries (besides Sherlock Holmes) but I enjoyed "Happy Birthday, Turk!"