November 2017, the official Scrum Guide was updated.
http://www.scrumguides.org/scrum-guide.html
See the “Parents of Scrum” Jeff Sutherland and Ken Schwaber discuss the updates, with lots of interesting background and color commentary.
https://www.scrum.org/resources/2017-scrum-guide-update-ken-schwaber-and-jeff-sutherland
The short guide itself and this video (runs just short of an hour) is fine primary source material.