Swedish journalist Anders Rydell wrote a book called 'Plundrarna: Hur nazisterna stal Europas konstskatter' a few years ago; with any luck it has been translated into a language you're comfortable with. It deals with how the nazis looted art prior to and during the war, but also devotes a few chapters to what happened after the war - dealing in artworks of dubious provenance, legal issues when heirs to the original owners wanted the looted art back &c.

Perhaps not quite what you had in mind, but most interesting and well researched.