The third part of the meditations (1599?) Book

I'm constantly in awe at the simultaneous boldness and compassion with which JPII tackles the taboo of human sexuality and love - as one who was raised in a Far-Eastern culture that almost never discusses this, even among Catholics, I found this work enlightening and full of rich lessons. His approach to love, both as a man and as a priest, is something that beats to a Divine rhythm - that of the Sacred Heart. I'll be digesting the contents of this work for years to come.

The one drawback -- it's not accessible to a wide audience. JPII's style is highly structured and can from time to time drift into academic-speak, which may make it seem somewhat unpalatable at times. Chris West does a good job bringing it to a broader audience in Theology of the Body Explained.