The Four Loves
Perhaps Lewis' most complex book, this one is an exhaustive analysis of love. He proceeds through the theme with almost scientific rigor, drawing parallels to literature and mythology, as well as biblical passages, to teach about affection, friendship, eros (the love between man and woman) and agape (or charity, the most sublime kind and mysterious of love).
One of the guiding threads of the book is its insistence that "when love becomes a god, it becomes a devil", that is, when a natural love strips man of all his principles and ethics, this love loses its divine image and becomes a kind of sick obsession. Lewis tries to show that human love is more love as it conforms to the divine will.
At first, it was thought of starting from the loves described by him (affection, friendship, eros and agape) and using simple images that could demonstrate the core of each type of love. However, the solution was very inconsistent with the images on the other covers in the collection.
A concept that had been developed in parallel was that of relating the "four" in the title to some image. The image of the human heart, with four chambers (two ventricles and two atriums) seemed the perfect choice. This image also matched the essence of the book which is this "scientific examination" of love. To further reaffirm the number four, the heart had its chambers differentiated with colors, one for each type of love.
On the back cover, the chosen image (which would be repeated in the chapter openings) followed what is said in the book: that human love is only love in fact when it has divine love as a component, when it is "under the cross of Christ". In the background, in the image of Doré, Virgil points up, as repeating to Dante the words of the apostle Paul: "God is Love".