I've been trying to relearn advanced math, having last touched it 20 years ago in engineering school (and then going into a non-technical career).  I've made it as far as differenrial equations.

I'm now going through the Differential Equations videos, but from the contents, I can see there aren't that many.

So I'm wondering if folks can recommend a textbook.  My biggest requirement is that it needs to be available in some form as an ebook.  Pdf, kindle, epub, doesn't matter.  Free or paid (up to maybe $300) doesn't matter. But my lifestyle means I rarely have time to read and study at home, and lugging around a physical text is not feasible.

Beyond that, I'd like something with a mix of theory and applications, especially electrical engineering applications.  I'd probably prefer there to be at least some theory - applications only tends to frustrate me.  I'd also like a book that's reasonably modern - I tried working with the much acclaimed Tennenbaum and Pollard, but kept getting bogged down by the language and format.  Something that teaches in a way compatible with Sal would be great.

ODEs to start with, put I'll probably want PDE eventually (not necessarily the same book).

Thanks!