Narada Bhakti Sutras by Swami Vivekananda

Bhakti is defined in the following Sutra. Those who have understood the magnitude of human sufferings in this sense-universe and those who have realised that this world is unreal, impermanent, illusory, perishable, full of troubles, difficulties, miseries, pain, sorrow and tribulation and those who desire to free themselves from the round of births and deaths with its concomitant evils of old age, diseases, etc., should know what Bhakti is, how to develop Bhakti, who is God or Lord or Isvara, the relationship between God and man and the methods to realise God, or to approach Him or to attain God-Consciousness.