Check out The 7 Essential Roles To Be the Best ScrumMaster by The 3Back Team. Here is an excerpt:
The ScrumMaster is seldom a ‘Master of Scrum;’ but the ScrumMaster’s primary responsibility is to help the Scrum Team use and improve its use of Scrum. This isn’t simply for Scrum’s sake; it’s because the Team needs to get better at doing what the Product Owner needs it to do….
The full article is available here.