Here in the community you can find a lot of topics with reference to quality materials. But here are some that I used and already indicated to other members:
I have a lot of documentation in portuguese, since im from Brazil, but here are some other links to material in english that may help you during your preparation:
:(A lot of tips for SM's)
- https://sites.google.com/site/wicmitchell/home (Blog from Ian, active member of the community), wich helped me a lot to understand some topics like Estimation and Sprint Planning.
: Collab.Net Youtube Channel with a lot of Scrum Training videos.
and of course, the 2 most mandatory:
- scrum.org : Share your doubts and learn with other peoples experience