I'm looking for references to study theory of ordinary differential equations. I'm looking for a similar book to Coddington's book, theory of ordinary differential equations but not this one, because this is a little old. I've already taken a course of (applied) differential equations but now I want to delve into the theory. I love Coddington's book but it is quite old. Also I like have more than one reference.
Thank you in advance for your time.