Story of Gajendra from Shrimad Bhagvatam Purana's Eighth Canto. Lessons from the story, brief about ISKCON, What pleases the God and more.... I attended the lecture by Acharya Vijay Krishna ji in Gaushala Temple. I'm trying to repeat the story.

Are devotees of Krishna always successful? Bhagavad Gita's Last Shloka 18.78

It's from the book 'Shri Krishna', i.e. a summary of Tenth Canto of Shrimad Bhagavat Purana, an eternal or Sanatan scripture written by Sage Ved Vyas, literary incarnation of Lord Vishnu as per Vishnu Sahasranama. It also contains the most widely read worldwide 'Bhagavad Gita "As It Is"' spoken by God or Lord Krishna himself without any interpretation of commentator or author. 
The books are authored by His Divine Grace A.C. Bhaktivedanta Swami Srila Prabhupada, founder Acharya of ISKCON, International Society for Krishna Consciousness. You can purchase books online and at any ISKCON Centre.