As an assistant, it's important to understand that "best" is a subjective term, but I would suggest two options depending on your background and how deeply you want to study the topic.

"Gravitation" by Charles W. Misner, Kip S. Thorne, and John Archibald Wheeler was first published in 1973 and was reprinted recently in 2017. This is an indication that the book is popular, but this book may not be the best for a newcomer. The publisher describes it as "landmark graduate-level textbook that presents Einstein’s general theory of relativity and offers a rigorous, full-year course on the physics of gravitation. Upon publication, Science called it “a pedagogic masterpiece,” and it has since become a classic, considered essential reading for every serious student and researcher in the field of relativity. This authoritative text has shaped the research of generations of physicists and astronomers, and the book continues to influence the way experts think about the subject."

As an alternative suggestion for a newcomer, I would also recommend "Introducing Einstein's Relativity" by Ray A. d'Inverno. Compared to "Gravitation", this text provides a gentler introduction to gravity and general relativity with more illustrations and a mathematical introduction to the concepts with less focus on the rigorous mathematical analysis. It is more appropriate for an undergraduate student.

Assuming these two questions are related, tensor analysis is an application of vectors and linear algebra to analyze physical fields, with the assumption that fields exist independently of one another. In terms of gravity (and relativity specifically), the "metric tensor" is the central mathematical description of the geometry of space time, commonly written as a 4x4 matrix. The book "Gravitation", recommended above, does include introductory chapters on tensor analysis and expands upon the techniques of tensor analysis throughout the book.
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