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 Lord is the creator of everything.
Bhagavad Gita 4.13,
To show them special mercy, I, dwelling in their hearts, destroy with the shining lamp of knowledge the darkness born of ignorance.
Bhagavad Gita 10.11,
Intelligence refers to the power to analyze things in their proper perspective, and knowledge refers to understanding what is spirit and what is matter.
Bhagavad Gita 10.4,
Because Bhagavad Gita is spoken by the Supreme Personality of Godhead, one need not read any other Vedic literature. One need only attentively and regularly hear and read Bhagavad Gita.
Bhagavad Gita Introduction,
Unless one reads the Bhagavad-gita in a submissive spirit, it is very difﬁcult to understand Bhagavad-gita, because it is a great mystery.