I'm looking for some good ODE/PDE textbooks that focus a little more on theory but that are still comprehensive in their respective subjects. I have taken ODEs and applied PDEs at my university but even though I got good grades, I feel like my knowledge is lacking. The books we used were Differential Equations by Polking, Boggess, & Arnold and Applied Partial Differential Equations by Haberman. I more or less want to start anew but in a more rigorous form. e.g., I took Calculus 1-3 using Etgen. However, I relearned everything with Courant & John and it really helped me read baby Rudin afterwards. I'd rather the textbooks not assume a lot of previous exposure to O/PDEs but a little is fine i suppose. any ideas??