Hello there!
After four months of very hard work, I am happy to announce the completion of a full course on I2C and SPI tailored to Arduino users.
This came about as the result of a successful crowd funding campaign that funded the development of a shield dedicated to I2C and SPI, as well as all the educational material to go with the shield.
As of now, there are 54 tutorials written, covering everything from what a register is (and how to build one with discrete 7400 series logic), up to driving an ADC using SPI at speeds that allow the sampling of audio frequencies.
I hope that the Arduino community finds this material of use, and I'm excited to be able to pay back to the Arduino forums after all the times I've scoured them for clarification on a topic.