Innumeracy by John Allen Paulos
Most people would be embarrassed to state that they were illiterate; why then do most wear innumeracy like a badge of honor? Partially it may be perception: when people think of math, they think geometry, algebra, or calculus. That's like thinking that being literate enough to read street signs is the same thing as taking an advanced class on English literature.
The author's fun, witty writing makes this potential dry subject fun. He identifies the basic elements of numeracy (spoiler: it's estimation techniques plus basic arithmatic) and demostrates their usefulness with examples. If you're one of those people (that is, most of them) who thinks that math is too hard and/or has no realworld use, read this book.
Other books by
John Allen Paulos:
Category  Title  Author  Rating  Description 

Mathematics  A Mathematician Reads the Newspaper  John Allen Paulos 

Paulos uses numeric wisdom to tear apart the myths and distortions that commonly appear in the daily newspaper. 
Mathematics  A Mathematician Plays the Stock Market  John Allen Paulos 

If you have any stock market investments, read this book. 