Thinking Fast and Slow is a pop psychology book, but one that leans on the academic side, detailing psychological experiments and applications of their consequences. Its main theme is how the assumptions of traditional Behavioural Economics, the main one of which is that people are rational, are flawed, and it gives many examples of results that show this. Here the use of the word rational specifically means that people will decide to do the thing that will give them a best chance of increase their utility, which from my understanding roughly means a measure of happiness. Often this behaviour is attributed to how System 1 and System 2 processes in the mind behave, which roughly mean "instinctual and instant" and "slow and conscious" respectively. System 1 is subject to a number of biases, and a number of experiments that reveal them are explained and discussed.
It's an extremely rich book, and one that I found quite eye opening. I thought I was fairly self-aware of the reasons why I do things, and that I am fairly rational. However, many times when reading I realised that I am guilty of the same irrational behaviour that everyone else seems to be. One the more self-help side, it gives suggestions on how to combat such System1 processes.
As a possible criticism, I'm unsure about the separation of all thoughts into 2 distinct categories. Perhaps in the future that will be revealed to be an over-simplification.