Without a doubt, L.A. Requiem is the finest thing Robert Crais has written, and I consider that high praise, as he is one of my favorite authors.
If you have been reading his books, Requiem will be a great payoff for you, since it brings to a head much of what has been building in the previous six Elvis Cole novels. If this is your first Crais book, rest assured you will like this book enough to want to go back and read the rest.
Despite Joe Pike being the character the plot is about, Elvis still remains central. Nearly every page is viewed through his eyes. Tha pages that deviate, when we see and learn about Joe's past, are among the most interesting in the book. Also, without giving too much away, the scenes with the killer are most revealing and in some ways creepy. Elvis remains aloof, but genuine, a much better protaganist than the typical Superman fantasy. Elvis can't beat up fifteen ninjas, pull a gun out of an extremity, shoot a sniper from 300 feet, and then anounce he has, in his head, completed the DNA test and found the real killer.
The one problem I would say I have with this book is it gives up too much. Too many things happen and when it is all said and done, in many ways, Crais has ruined the series. The follow-up, The Last Detective, felt lacking after the carnage of this entry, naked without much of the mystery and buildup. Thankfully, it still tackled a major piece of character development, but left the series with absolutely no steam for the next book. It is possible you could simply read the series and consider "Detective" the final chapter.
Also worth checking out is Crais's Hostage, soon to be made into a movie by Bruce Willis. It is a great stand-alone novel.