Erich Fromm's most influential book is generally considered to be The Art of Loving, originally published in 1956. Fromm's (1956) most radical departure from common lore was to present love as an activity, not a mysterious and short-lived sensation. Hence, Fromm (1956) posited that love can, with skill, be developed as a permanent state.
Reference
Fromm, E. (1956). The art of loving. Harper & Row.
The Heart of Man: Its Genius for Good and Evil, originally published in 1964, is another influential book by Erich Fromm in which he reflected on and analyzed the capacity of human beings when it comes to love of life or death, violence and frustration, seeking pleasure or causing pain, etc.
Reference
Fromm, E. (1964). The heart of man: Its genius for good and evil. Harper & Row.