The Nature of Happiness by Desmond Morris, is a brief little book describing the author's beliefs about various types of happiness.
I think it's worth including in a list of books to read about happiness, though I find it to be quite a superficial opinion piece. There are a few interesting observations; some sound, simple advice; and a collection of nice quotations from famous authors, but otherwise the book really lacks substance. There is almost no reference to research; there are many sweeping statements, such as about evolutionary underpinnings of happiness-related behaviour, yet without a rigorous development of these ideas, and perhaps without a sense of understanding the voice or perspective of those to whom he is referring.
It is always surprising to me how a minor text of this type could warrant a glowing review from a major newspaper:
"At last, a highly intelligent, serious exploration of a subject as universal as it is mysterious...an illuminating and fascinating read." The Times