Very accessible introduction on chaos theory. Style is informal and nontechnical. You're not required to have a particular background in mathematics to understand the content. Reading it is particularly easy, and it provides excellent pointers to more advanced topics on chaos theory.
If you already have some background in dynamic systems and modeling, i would recommend Morrison's book : "The Art of Modeling Dynamic Systems : Forecasting for Chaos, Randomness, and Determinism" which is somewhat more technical but certainly more helpful for getting a good idea of what chaos theory is, at an engineering point of view.
In addition, for getting a very broad view of chaos in the real world, Schroeder's "Fractals, Chaos, Power laws: Minutes from an Infinite Paradise" is incomparable ! It overwhelms you with the footprints of chaos in every possible context. A must-read ! But very technical and requires some important knowledge in physics and science engineering...
However, as an easy nontechnical introduction on chaos theory, this book really deserves its 5 stars !