Srila Prabhupada said that if we read four of the books he translated and commented upon, that is sufficient to attain perfection and to return back home, back to Godhead. These four books are the Bhagavad-gita, The Nectar of devotion, Srimad-Bhagavatam, and Caitanya-caritamrita.
Of these four the Bhagavad-gita and the Nectar of Devotion represent the basics of the philosophy of Krishna consciousness. If in the beginning one has a hard time to grasp the meaning of these books, there are smaller books that present the teachings of the Bhagavad-gita in a simplified way; so one can start from them. Such books, for example, are The Nectar of Instruction, The Perfection of Yoga, Perfect Questions, Perfect Answers, Beyond Birth and Death, Chant and Be Happy, The Science of Self Realization, etc.
The Bhagavad-gita is dealing with all the major philosophical topics like the various types of yoga and how to practice them, the laws of karma and reincarnation, God, the living entities, the material nature and the connection between them, the eternal time and so on. The Nectar of devotion describes in detail the process of bhakti-yoga (Krishna consciousness).
After one masters these subject matters, one can go deeper in the science of bhakti by reading Srimad-Bhagavatam and Caitanya-caritamrita. These two books are considered postgraduate studies in the spiritual science.
Srimad-Bhagavatam is known as the ripened fruit of the tree of the Vedic literature. It describes the essence of spirituality in a story form. The main topics of Bhagavatam are the creation and annihilation of the universe, the duration of its existence, the law of karma, how the Lord protects His devotees, the main demigods and rulers of the universe, liberation from material existence and the ultimate shelter of all.
Sri Caitanya-caritamrita describes the life and activities of Sri Krishna Caitanya Mahaprabhu, the last incarnation of God. He introduced the sankirtan-yagya, the congregational chanting of the Holy names of God.
Another important book is Krishna, the Supreme Personality of Godhead. Srila Prabhupada recommended reading it before going to bed. This is the retold version of the Tenth Canto of Srimad-Bhagavatam and describes in details the activities of the Supreme Lord Krishna when He was present on the planet Earth 5000 years ago.
Additionally, there are many books written by Srila Prabhupada’s disciples or other Acharyas before him, that are deepening our understanding of the above mentioned four books. One may read them occasionally, but the four main books (the Bhagavad-gita, The Nectar of devotion, Srimad-Bhagavatam, and Caitanya-caritamrita) one should try to read every day. Daily reading and studying these books will make us strong spiritually just as one gets stronger by proper eating. As the food nourishes the body the knowledge of these books nourishes the soul.