Essential Scrum – A Practical Guide to the Most Popular Agile Process
1st Edition, 2013
Essential Scrum by Kenneth S Rubin is a solid piece of work. It covers (and often repeats coverage of) Agile, especially the Scrum approach in a detailed and quite knowledgeable manner.
He not only provides an overview of Scrum, but digs in deeper covering the topics in detail. He even discusses alternatives (for example, do you commit to a Sprint Backlog or do you forecast the work attempted?)
He has clearly researched his material, with citations liberally sprinkled throughout the text.
I recommend this book for organizations that are just now starting Scrum as it encourages the fledgling practice in shops that are new to the concept.