Having read a pile of behavioural economics, here's another. It's something of a pop behavioural economics classic. At 260 pages, it's much shorter than Thinking, fast and slow, and in terms of content, it's shorter still. As the sales pitch for libertarian paternalism, it's got some nice examples, and is a very straightforward read.
Had I not read around the subject before, I'd probably find it more interesting, but it didn't really bring much new to me. If I had to become a 'choice architect' it would provide some nice inspiration. If I wanted to recommend an introduction to the subject, this would be it. As it was, I found it a little dull.