Connelly has never written a bad book (although Chasing the Dime, a stand-alone thriller, is below the rest in quality), and all of his Bosch books are worth seeking out (they should be read in order). But for those who have yet to discover him and have time on their hands this summer, I would recommend the following arc:
The Black Echo, which introduces Bosch and Eleanor Wish, who plays a critical role in his life and the development of his character.
The Poet, a non-Bosch book which introduces F.B.I. agent Rachel Walling, and the serial killer known as "The Poet."
Trunk Music, featuring the return of Eleanor Wish, the introduction of a new partner (Kizmin Rider), and a plot development which changes Harry's life.
Blood Work, a non-Bosch book which introduces former F.B.I. agent Terry McCaleb, who is recovering from a heart transplant and asked by the sister of a murdered woman to help in solving the case.
A Darkness More Than Night, feauturing both McCaleb and Bosch, walking the fine line between being partners and being antagonists.
City of Bones, which concludes with another major plot development that changes Harry's life.
The Narrows, in which it is revelaed that Terry McCaleb has died, and in seeking to understand Terry's death Bosch comes face to face with The Poet. In the process, he meets and joins forces with Rachel Walling.
Both of Connelly's most recent Bosch books, Echo Park and The Overlook, also feature Walling with Bosch.
Having read all of Connelly's books, I'm comfortable in saying that he, along with Robert Crais, is the pre-eminent writer of crime fiction working today.
A sample of his writing, from the beginning of The Narrows (my favorite of his books):
I think maybe I only know one thing in this world. One thing for sure. And that is that the truth does not set you free. Not like I have heard it said and not like I have said it myself the countless times I sat in small rooms and jail cells and urged ragged men to confess their sins to me. I lied to them, tricked them. The truth does not salvage you or make you whole again. It does not allow you to rise above the burden of lies and secrets and wounds to the heart. The truths I have learned hold me down like chains in a dark room, an underworld of ghosts and victims that slither around me like snakes. It is a place where the truth is not something to look at or behold. It is the place where evil waits. Where it blows its breath, every breath, into your mouth and nose until you cannot escape from it. This is what I know. The only thing.
I knew this going in on the day I took the case that would lead me into the narrows. I knew that my life's mission would always take me to the places where evil waits, to the places where the truth that I might find would be an ugly and horrible thing. And still I went without pause. And still I went, not being ready for the moment when evil would come from its waiting place. When it would grab at me like an animal and take me down into the black water.
And there, in a nutshell, you have Harry Bosch.