The Tiger that Isn't is a new book by Michael Blastland and Andrew Dilnot, about how we (non-mathematicians) can make sense of the numbers we see every day, especially in the media. I haven't read the book yet, but the authors have a nice essay on the basic ideas that is worth reading. It turns out that common sense can help us see through a lot of the numerical claims that surround us:
"We all know more than we think we do. We have been beautifully conditioned to see through numbers, believe it or not, by our own experience. Numbers can make sense of a world otherwise too vast and intricate to get into proportion. They have their limitations, no doubt, but are sometimes, for some tasks, unbeatable. That is, if used properly."
The essay is worth reading, even - or especially - if you have no interest in the book itself.