Bhagavad-gita is the instruction given by Krishna, and Srimad-Bhagavatam is the book containing topics exclusively about Krishna and His devotees. These two books are the special nectar of the words of Krishna.
Srimad Bhagavatam 4.4.44,
The Bhagavad-gita can be consulted in all critical times, not only for solace from all kinds of mental agonies, but also for the way out of great entanglements which may embarrass one in some critical hour.
Srimad Bhagavatam 1.15.27,
To manage a sankirtana party nicely is more precious than all other activities. That is the verdict of Bhagavad-gita and the mission of Lord Sri Chaitanya Mahaprabhu.
Letter to Tamala Krsna – Bombay 13 November, 1975.