Off on a Tangent: Meditations on the derivative is an in-progress open-source book(?) about incarnations of the derivative: derivatives of single variable functions, derivatives of multivariable functions, and pushforwards of tangent vectors along smooth maps of manifolds. You can find a PDF of the most recent version in this repository.

I last used it to teach a second course in real analysis at Colorado College in Fall 2019. The text is somewhat incomplete, especially towards the end, but I hope to complete it someday. If you find yourself using it (or thinking about using it), please tell me about it! Maybe hearing about that will motivate me to finish it :)

The text is copylefted under CC BY-SA 4.0.