I recently participated in a group study through the beatitude section of Martyn Lloyd-Jones' Studies in the Sermon on the Mount. I read a wide range of other material while we worked through this book. You can't do much better than this book for practical insight and interpretative helps. Most of the books I read written after this publication draw heavily on ML-J's insights. So read this section, and you will have most of what was written after it.
Let me also highly recommend John Piper's sermons. Piper does a wonderful job keeping the context in view while pointing out the significant cross-references. I found Piper just as helpful as ML-J, and sometimes he may have had the edge. Quite an admission coming from me!
Lastly, I enjoyed reading some of the insights from Thomas Watson. I wasn't able to read him as much as I had hoped, but was encouraged when I did. Only a Puritan could pull off 300+ pages on 12 verses!
I believe there is still much work to be done in our understanding of this section. The most glaring omission I found was the lack of consideration concerning how these characteristics were manifested in the life of Christ. These are often mentioned, but never developed. There is one author who sought to give consider this thought, but I believe more could be done.