Introductory text on perturbation theory for dynamical systems

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I am working on my thesis which is about oscillations and as far as I realise I need to know about perturbation theory and methods in solving differential equations, specifically dynamical systems. A Googling returned the following results:

Does anybody have any suggestion? First one seems reasonable but I am not sure about the quality. Second and third one is very general. Forth one is only about finite dimensional linear operators and quite long. What I am looking for is a concise and short introductory text for perturbation theory in Dynamical Systems.