I just graduated from an engineering school and am about to begin classes as a PhD candidate. As I review my undergraduate classes and sift through old notes, I find myself wanting for more math knowledge. While I did well in math classes, I always felt as though I was following steps rather than gaining a depth of understanding and intuition. To get to the point:
Can you recommend any books to brush up on and perhaps gain a more advanced understanding of calculus and PDEs?
As an undergraduate, I used Salas Hille and Etgen, which I felt was great at teaching rote steps to solve problems but lacked some depth.
Thanks in advance!
(Questions regarding a reasonable self-guided book have very frequently been brought up on this forum. However, I like to think this is unique and apologize if this feels "repeated.")