I am an undergraduate in Mathematics writing his dissertation on General Relativity this year. The next couple of months will be dedicated to learning the math (geometry) and the physics, and to this end I am looking for must-read/must-watch material on the topic.

General book recommendations are welcome, but keep in mind I can use Google! I am mainly looking for really good introductory content that I would have a hard time finding myself, whether it be videos, course notes, books or blog posts.

My apologies if this is a duplicate!