Reddit mentions of Redemption Accomplished and Applied

Preface

Unfortunately I don't think there's a "one stop shop" book on the atonement. The atonement has a biblical/exegetical basis as well as philosophical and theological ramifications. Hence, to do justice to the atonement as a full-orbed topic in a single book, you'd have to find a scholar who is well versed and up-to-date in the relevant biblical scholarship as well as philosophical theology. That's exceedingly rare, and in fact I'm not sure if there is such a scholar today.

What's more, the atonement can be framed in terms of additional categories or sub-categories like biblical theology, Pauline theology, Johannine theology, and so on. (By the way, Tom Schreiner, Simon Gathercole, and Jarvis Williams are good in discussing the atonement in Pauline theology.)

In short, the atonement is a massive topic.

One book

However, if I had to pick a single book on the atonement that gets as close as possible to this ideal (but ultimately falling short of it), I think I'd recommend Pierced for Our Transgressions. The book has decent biblical/exegetical and theological (including historical theology) foundations. Not stellar in these categories, but not bad, solid. However, it significantly lacks in philosophical theology. In any case, I think you'd have to supplement this book with other books. I'd recommend:

Biblical/Exegetical

• Beilby, James and Eddy, Paul (eds.). The Nature of the Atonement: Four Views. This book is a debate between four scholars arguing for four different views on the atonement: Gregory Boyd argues for the Christus Victor view; Tom Schreiner argues for the penal substitutionary view; Bruce Reichenbach argues for the healing view; and Joel Green argues for a kaleidescopic view. In my view, Schreiner makes the best case, but read it for yourself to decide.
  
  
• Morris, Leon. The Apostolic Preaching of the Cross. Either this book or its less technical and more popular but still strong treatment The Atonement: Its Meaning and Significance. This is an older text, but it was and remains a landmark text. D.A. Carson still tells seminarians and ministers to "sell your shirt and buy" Morris' book if they have to. Likewise see Morris' brief essay "Theories of the Atonement".
  
  Theological
  
  
• Murray, John. Redemption Accomplished and Applied. A classic Reformed text from a stalwart Reformed theologian.
  
  
• Nicole, Roger. Our Sovereign Savior. A good chapter on the atonement by a world class theologian. Nicole was a Swiss Reformed theologian.
  
  
• Nicole, Roger. Standing Forth: Collected Writings of Roger Nicole. Includes essays on the atonement and related matters.
  
  
• Packer, J.I., Dever, Mark, and Duncan, Ligon. In My Place Condemned He Stood: Celebrating the Glory of the Atonement. This book contains several essays on the atonement from J.I. Packer including Packer's classic introduction to John Owen's The Death of Death in the Death of Christ.
  
  
• Warfield, B.B. "Atonement". A brief article that gives an overview of five possible theories about the atonement. Warfield has other good material on the atonement, but I thought this would be a decent representative.
  
  Philosophical
  
  
• Craig, William Lane. The Atonement (in the University of Cambridge's Elements in the Philosophy of Religion series). A short book. An overview of the philosophical issues. Cambridge Press did offer it for free as a downloadable pdf, which is how I obtained it, but I don't know if that's still the case now.
  
  
• Helm, Paul. "John Calvin's Position on the Atonement". Free article from an astute Reformed philosopher. Helm has discussed the atonement in published books too.
  
  
• Helm, Paul. "The Logic of Limited Atonement". Another free article.

Yes, I do. As does /u/bsmason

Here's a hit list.

Episode 200 of Christ the Center. Richard Gaffin Jr. and his student Lane Tipton in particular are the best. If you are not familiar with Christ the Center, check them out. They also did a whole conference on Union with Christ. You will also find a debate between Horton and Tipton on this topic.

One With Christ

The Chapter on Union with Christ in John Murray's Book, Redemption Accomplished and Applied.

Sinclair Ferguson has now written on it in his excellent and recent book The Whole Christ. This one is good because he speaks on the dangers of Legalism and Antinomianism, which are the resultant errors when we get a function theology of Union wrong.

Tangentially related: Mark Jones' Antinomianism, Reformed Theology's Unwelcomed Guest is also related to the topic. He is particularly strong on Christology, and consistently emphasiszes that we are united to Christ, the God-Man and how this should impact us.

I have read all these books, some more than once, and recommend them.