My Dad gave me a most of a series that he's been reading for me to read as well. The difference between my Dad and me is that I have to read the series in order, and he doesn't. This book was not included in the bundle, as he'd borrowed it from another library while on vacation, and so I got it from my local library.
This book is definitely an adult selection. Were this book to have a warning label, it would be labeled for violence, language, and mature content. The star of the book, and of the series (this is book one), is Harry Bosch. He is a vietnam war veteran and a homicide detective in the Hollywood division of LAPD. He is a very complex and well-crafted character, as are all the characters in the novel, including the victims. He is not the perfect hero as he has significant failings, flaws, and vices. The reader is asked to overlook these, with the commendation that he is extremely good at his job, and indeed it is easy to overlook them.
In this novel, Harry finds an old acquaintance from Vietnam dead in a tunnel. At first glance it looks like an overdose, but Harry suspects more is at stake. He digs in, despite resistance, and uncovers, well, many interesting things. I can't say much more without revealing the good stuff, but I can say that if you like detective novels, this is a great read. While it's not a slow read, there are many seemingly inconsequential points that don't pull together until the final quarter of the mystery.