Literature on differential equations?

Question

Books

Please tell me a good and not difficult to understand literature on the basics of differential equations.
What the lecturer suggested:

1. L. E. Elsgolts - "Differential equations and calculus of variations"
   
2. A. M. Denisov, A. V. Razgulin - "Ordinary differential equations"
   
3. I. G. Petrovsky - "Lectures on the theory of ordinary differential equations"
   

I chose the second one for now, since the book came out relatively recently.

Answer 1

new.math.msu.su/diffur/main_du_ast.pdf