Although there are probably several reasons for the shortage of psychological research concerning humor, one such reason no doubt involves the fact the past research tended to consider humor to be a one-dimensional characteristic. On the other hand, Martin argues that there are four distinct types of humor to consider: Affiliative (positive, facilitates relationships); Self-enhancing (positive, using humor to cope); Aggressive (criticize or manipulate others); and Self-defeating (self-disparaging). The distinction seems to me a distinct and significant step forward in humor research, and studies using Martin's resultant Humor Styles Questionnaire (HSQ) are already bearing this out. I can honestly say that his book made me think about how I use humor, and as a result influenced both my teaching and my social interactions.
In addition, among a variety of related topics Martin covers the history of humor research, major theories, the development of humor, and correlates of the use of humor (physical and mental health, use in education, etc.). I was very impressed with this book (and I've obsessively read roughly one hundred books in the past year related to positive psychology), and have been recommending Martin to anyone who is interested in humor and who will listen.