Scrum Guide (scrumguides.org)
What: This is the root of Scrum. The core description of Scrum, including the Roles, Rituals and Artifacts. It is compact – still under 20 pages – so it is very accessible. No worries if your English is not profound, it is available in many translation.
Why: Before reading anything else in regard to Scrum. Stop. Read the Scrum guide first. All materials, books and articles available continue to build upon this description. Even when once read; make sure to re-read it occasionally.
Who: It has been written and maintained by Scrum-founding-fathers Ken Schwaber and Jeff Sutherland.