Sarva-samvadini is a supplement to the Ṣaṭ Sandarbha of Śrī Jīva Gosvāmī. In this book Śrī Jīva Gosvāmī adds extra information on certain topics discussed in the Sandarbhas. It is in this book that he names the philosophy and theology of Śrī Caitanya Vaiṣṇavism as Acintya-bheda-abheda. The language of the book, however, is highly philosophical. Paṇḍita Ānanda Gopāla Dāsa of Jīva Institute has written a lucid commentary on it in Sanskrit. This is the maiden commentary on this important work. It is a must for those who really want to dive deep into the siddhānta of Caitanya school
Bhagavatam is the top most Purana. It is called Bhagavat Purana because it is Bhagavan himself. It comes from him directly. This Bhagavat Purana is Krishna. It is he himself. That is why Jiva Gosvami is trying to establish this fact that to understand the supreme goal of life, you should study this book.