Bhakti Yoga by Swami Vivekananda is an introduction to Bhakti Yoga. From the foreword:
“Bhakti Yogais a real, genuine search after the Lord, a search beginning, continuing, and ending in Love. One single moment of the madness of extreme love to God brings us eternal freedom. “Bhakti,” says Nârada in his explanation of the Bhakti-aphorisms, “is intense love to God.”—“When a man gets it, he loves all, hates none; he becomes satisfied for ever.”—“This love cannot be reduced to any earthly benefit,” because so long as worldly desires last, that kind of love does not come. “Bhakti is greater than Karma, greater
than Yoga, because these are intended for an object in view, while Bhakti is its own fruition, its own means, and its own end.”