This seems a very good and in-depth. I recently had to learn a bit of this stuff and coming from very basic math Quats are a bit of funny concept at first. This

This seems a very good and in-depth. I recently had to learn a bit of this stuff and coming from very basic math Quats are a bit of funny concept at first. This video does an amazing job at explaining what they are https://www.youtube.com/watch?v=4mXL751ko0w https://www.youtube.com/watch?v=4mXL751ko0w and the pro/cons over euler angles. The whole channel has some pretty good resources for anyone looking for a "soft approach" at math.