Swami Vivekananda 
Bhakti yoga 

This book presents clearly and concisely the principles of Bhakti yoga and the practical formulas needed to practice it. Bhakti yoga is the path of devotion which unites the practitioners finally with God Almighty. Unless bhakti is practiced, karma and jnana will lead only to unfruitful knowledge – knowledge without joy.
This book is an attempt to bring the principles of Bhakti yoga to the knowledge of the common reader. Practical formulas, based on the age-old experiences of the saints, are given, which can be followed by the practitioner to become one with God Almighty.
The methods are based on my personal experiences over the years, and there is nothing which has not been tested by me personally