Another +1 for A Random Walk Down Wall Street
I was originally going to suggest Happy Money, but might be too simplistic? It's a really easy read, but they do get into studies and the like, looking different ways of spending money and how they affect happiness. Some are relatively common knowledge (at least around these parts of the internet), like how fancy houses don't make you happier. Other bits of observations from research are less intuitive, like pre-paying for experiences. My husband and I both really enjoyed that book, and we're both engineers.