Bhakti-rasamrta-sindhu is a book written by rasikacarya Srila Rupa Gosvami in an elaborate way about the science of devotional service, and by reading this book one can understand the meaning of devotional service. But to elaborately explain madhura-rasa, which is not dealt in great depth in Bhakti-rasamrta-sindhu, he wrote Sri Ujjvala-nilamani. It deals with every aspect of madhura-rasa. This book is the only definitive guide to madhura-rasa and ragamayi-bhakti.
Caution: This book will not be comprehensible unless one dives into the deep ocean of bhakti, i.e. Bhakti-rasamrta-sindhu. Only after attaining proficiency in Bhakti-rasamrta-sindhu should one dare to search for this blue emerald, Ujjvala-nilmani, found deep within the ocean of bhakti, otherwise one may get lost. We should very carefully understand the exalted position of this book and also appreciate the qualification that are required for hearing it.