In Pursuit of the Unknown by Ian Stewart, 2012, ISBN 978-0-465-02973-0. Subtitle: 17 Equations That Changed the World. This book has a label on the back the says Mathematics, but I think it should be labelled Physics, etc. It is really more about the subjects of the equations than the mathematics. Subjects discusses include Pythagora's Theorem, Logarithms, Calculus, Newton's Law of Gravity, Square Root of Minus One, Euler's Formula for Polyhedra, Normal Distribution, Wave Equation, Fourier Transform, Navier-Stokes Equation, Maxwell's Equations, Second Law of Thermodynamics, Relativity, Schrodingers Equation, Information Theory, Chaos Theory and Black-Scholes Equation.
If you are interested in how mathematics is used in the real world, I think this is a good introduction. It is well written, informative and I think entertaining. Sadly there is no list of books for further reading.
 Purchased from Amazon on 2013-3-7 for $20.51.

The Pythagorean Theorem by Alfred S. Posamentier.  2010, ISBN 978-1-61614-181-3. Subtitled: The Story of Its Power and Beauty.
The cover shows a "Pythagorean Tree" constructed from layers of Pythagorean triangles with squares on their sides. (Discussed in chapter 7.) This book is similar to the book by Eli Maor, but covers topics not discussed in that other book. Both books are worth reading.
Purchased from amazon.com, 10-5-2010, for $19.71.

The Pythagorean Theorem by Eli Maor. 2007, ISBN 987-0-691-12526-8. Subtitled: A 4000 Year History. Excellent book. I really enjoyed reading this. The subject is handled correctly and the author doesn't shy away from doing real math, ie, providing theorems and their proofs. However, the book is mostly about the history of this idea and how it has come to be used in various areas of mathematics and science. I will be reading his other books soon. Purchased at Powell's books on 7-27-2007, 24.95.