The problem with starting with papers is that this is a a very complex and controversial topic (no different than anything else in psychiatry- or medicine for that matter), and a Nora Volkow review from 2005 might be completely contradicted by a Nora Volkow review from 2010. A lot of the original work was done in murine models and Pet scans, and different fMRI techniques seem to constantly build on the reward/motivation story. Now with optogenetics and the ability to target specific cell populations, what was speculative can now be confirmed (at least in rodent models) Karl Diesseroth's lab just recently published a paper in Science demonstrating the mPFC exerting top down control over the striatum, which was strongly suggested but never proved.
Anyway, for a beginner I like the chapter on addiction from Stahl's Psychopharmacology, though realizing the limitations that it is overly simplistic, speculative, and poorly referenced. However, it's a place to start, and then you can search for reviews from Nora Volkow, George Koob, Mark Gold, Eric Nestler, etc