A short history of the Narada Bhakti Sutra:
In 1967, His Divine Grace A.C. Bhaktivedanta Swami Prabhupada translated and wrote purports for thirteen of the eighty-four aphorisms (Srila Prabhupada called them “codes”) of the Narada-Bhakti-Sutra.
In 1989, at the annual GBC meeting, they suggested, that Satsvarupa and Gopiparanadhana Prabhu complete this Narada-bhakti-Sutra. They were happy to accept the assignment, especially because of Satsvarupa’s involvement with Srila Prabhupada’s initial writing of the Narada-bhakti-sutra.
When Srila Prabhupada left New York early in 1967 and went to San Francisco, Satsvarupa wrote Prabhupada a letter asking if he would translate the Narada-Bhakti-Sutra. Srila Prabhupada’s reply, dated February 10, 1967:
“Yes, please send me immediately one copy of Bhakti Sutra (with original Sanskrit text). I shall immediately begin the commentary.”
At first Prabhupada’s translation of the Narada Bhakti Sutra went quickly. He sent tapes of his dictation in the mail, and Satsvarupa transcribed them along with the tapes sent for Prabhupada’s major work, Teachings of Lord Caitanya. From the beginning it was understood that Narada-bhakti-sutra was a kind of “extra” for Srila Prabhupada and thus he was never able to personally complete it.
Then, in March of 1967, while Srila Prabhupada was still residing in San Francisco, he wrote Satsvarupe this letter:
“Please accept my blessings. I have seen the typed copies of Narada Bhakti Sutras as well as Teachings of Lord Caitanya. Both of them are nicely made. I think let us finish first Teachings of Lord Caitanya and then we may take again Narada Bhakti Sutras. The subject matter discussed with Narada Sutras is already there in the Teachings of Lord Caitanya.”
So Srila Prabhupada’s work on the Narada-Bhakti-Sutra stopped, and was never resumed in order to concentrate on finishing the Teachings of Lord Caitanya.
NOTE: Although this book cannot be said to be an original book of Srila Prabhupada (because it was not completed before Srila Prabhupada left, and he did not personally work on the entirety of this book) he did authorize and bless its publication, so it is included here for your own reference for free download.