Books and materials.

Here are links to the two most important documents in the world of Scrum and Agile.
The Scrum Guide by Jeff Sutherland and Ken Schwaber contains the agreed upon definition of the Scrum Framework. It is a living document and has been updated repeatedly. The latest update was made in November, 2017.
The Manifesto for Agile Software Development (aka "The Agile Manifesto") was first concieved in the winter of 2001. It contains the four fundamental values and twelve fundamental principles of Agile software development. It has never been changed or updated since it was initially published.
Here are links to the Amazon.com listings of several important books on the Scrum Framework.
Despite its title, Navigating Hybrid Scrum Environments by Frederik M. Fowler actually describes the Scrum Framewoprk in great detail. It explores the components of Scrum and explains their purpose in the Framework. It is focused not on "what" the components of Scrum are but "why" those components are there in the first place. It discusses the effects that can be expected if parts of Scrum are left out in "hybrid" environments.
Software in 30 days by Ken Schwaber is one of the first books ever published on Scrum. It is still one of the best. It is "must" reading for anyone wishing to understand and master the Scrum Framework. Ken Schwaber is the co-developer of Scrum and his insights are key to understanding it.
Scrum: the art of doing twice the work in half the time by Jeff Sutherland is a recent book by the other co-developer of the Scrum Framework. It contains a number of insights including Jeff's thoughts on how to "scale" the Scrum Framework.
Agile Product Management with Scrum by Roman Pilcher is one of the first (and best) books about Scrum Product Ownership. Pilcher described his own experiences from this own point of view and shares many insights into the role of the Product Owner in Scrum.
Coaching Agile Teams by Lyssa Adkins is one of the best books on practical Scrum Mastery available. Lyssa actually coined the term "Agile Coach" with this book.