Recently, a person from Australia asked me to recommend which books to read on the science of bhakti (devotion), and I suggested the four texts that our guru, Prabhupada, encouraged his students to study. They are Bhagavad-gita As It Is, Bhagavatam, Nectar of Devotion, and Chaitanya Charitamrita (or Teachings of Lord Chaitanya).
I said in my correspondence that if you get these publications down, familiarizing yourself and applying the practices, you are doing very well in the devotion category