Can anyone compare this vs other resources to learn differential equations? I want to learn math roughly to the level of an undergrad engineering student, so I've looked at some Advanced Engineering Mathematics books (one by Zill, another by Kreyszig), both have mostly good reviews, and to be honest, Paul's Notes seem almost a level above in clarity and understandability. For example, compare the explanation of integrating factor and exact equations. The books typically explain exact equations via a total differential, which I'm kind of confused about... why not just say that the left part of the equation is a total derivative?

I can't decide whether to continue reading the Advanced Engineering Mathematics book or learn the topics it contains via Paul's Notes and other resources. My worry is that the reason Paul's Notes seem clearer is simply because they're more superficial.

What are some other good learning resources for advanced engineering undergrad math?