The Two Sides of Love: What Strengthens Affection, Closeness, and Lasting Commitment by Gary Smalley and John Trent

The authors use very simple but powerful concepts to cover four main personality styles.  One of the main principles it explores is "how people's natural personality strengths can become their biggest weaknesses and most irritating behaviors".  Understanding how this works helps the reader to become more softer or tender in their loving or more consistent and "harder" as needed.