General Presentations
http://doctoryes.github.io/openedx_talk_xblocks/#1
XBlock API Guide
http://edx.readthedocs.org/projects/xblock/en/latest/
XBlock Tutorial
If you haven't yet gone through the tutorial, do so!
http://edx.readthedocs.org/projects/xblock-tutorial/en/latest/
Braden MacDonald has also made a great presentation for OpenEdxCon 2016 on making an XBlock: