One addition to Crossway's republication of Love Your Enemies is a new preface from John Piper in which he explains why this book is a monument to God's mercy and faithfulness. He writes,
Monuments are usually built much later than the event celebrated, in this case, the years 1971–1974 in Munich, Germany. Monuments are not about themselves. They remind. They point. In this case, to the mercy and faithfulness of God. Monuments are often made of plain, lifeless stone while representing something utterly vital and beautiful. In this case, a mere book representing the shining face of God and his mighty hand on my life in those days — and before. . .
To read the entire preface, or the entire book, get the free download: Love Your Enemies: Jesus' Love Command in the Synoptic Gospels & the Early Christian Paraenesis (PDF).