This summer I worked in an investing firm, and figured it was worth learning something about behavioural finance before starting. This book is nice and short, but packed full of studies and summaries of findings in the literature on cognitive biases. If you have read anything from this field already (e.g. Thinking, Fast and Slow, #1 below), then a good portion of the biases and studies Montier highlights will be familiar. However, even given this familiarity, Montier does a good job of showing how these biases can be applied specifically to investing, and which you should be most vigilant to watch out for as an investor.
Overall: nice and short and to the point.