Browning is an outstanding historian on the Holocaust and the origins of the Final Solution. He is a very good place to start. For other historians and modern scholarship, I would also definitely recommend Peter Longerich. Also, Nikolaus Wachsmann has just released a book, KL, which deals with the concentration camp system (as distinct from the extermination camps) and its role within NS political and social policies. While this may not deal directly with the Final Solution, it might give you a good look into how and why camps were conceived in the 1930s by the NS, and how these were adapted to fit not only expanding populations but also expanding usage.