Right now Logos Bible Software is a great deal for two important books on the topic of Justification: 

Get this book for free: Justification Reconsidered: Rethinking a Pauline Theme by Stephen Westerholm (on Amazon it is currently $11.94) 

Then get this book for just 99¢: The Deliverance of God: An Apocalyptic Rereading of Justification in Paul by Douglas A. Campbell (on Amazon it is currently $49.50)

If you want to be up to date on the status of ‘Justification’ in the last decade or two, these books will go a long way! Get a survey and critique of some of the key players in Pauline scholarship with Westerholm’s book. And dig deep down to have some of your closest held presuppositions on Paul’s theology challenged with Campbell’s tome on ‘Justification’ (also the longer of the two books by over a thousand pages!).