A friend recommended this book, so I thought I'd give it a read. I was really impressed with the spirit of the book and I'd definitely recommend it. I felt like there were two themes to the book:
1) Correlation does not imply causality. People often find a correlation between two or more things and assume that correlation demonstrates a causal relationship between those things. This isn't necessarily true and there are often many other factors that need to be considered.
2) Asking "Why?" can help us discover very unusual and interesting things. Accepting opinions and hypotheses without scrutiny can lead us to false conclusions and rob us of genuine understanding. Repeatedly asking "Why?", even when the answer seems obvious or at least trivial, can help us uncover interesting relationships between seemingly unrelated things.
It was a pretty quick read, but it helped me see that I should probably be thinking deeper and more critically than I typically do.