Russ Harris is a Melbourne based GP, best know for his passion for ACT (accept
ance and commitment therapy). What is ACT you ask. It is a type of therapy, related to cognitive behavior therapy, though is some ways quite different. I plan to give a more complete summary at a later date, but it’s most interesting strategy involves not eliminating or arguing with thoughts or feelings but just deciding if they are useful. This aspect is actually quite different from CBT, which often involves assessing beliefs empirically to decide if they are accurate. It may seem a minor point, but in treatment if can be very significant.
If you have previously failed CBT or other therapies, in many instances trialing ACT may be of value. The Happiness Trap was Russ’s first book. It really is a great book from a purely clinical point of view, though as a piece of literature it is probably not his best work. His later books are better written from a literary style point of view, though the Happiness Trap probably remains the best and most concise of his books on ACT.
What I like most about the book was the middle chapters on dealing with intrusive and obsessive thoughts. I think 
the techniques are especially useful for over thinkers and obsessive perfectionists, for whom CBT often doesn’t work. This is the section I most recommend to people.
The Third part of the book is reasonably standard goal setting. It is useful, but there are probably better books out there for general self help.
The Happiness trap is now available in a pocket book, which I have had some good feedback about. Russ has written further books since then, including the The Confidence Gap, The Reality Slap and ACT Made Simple. The are certainly all worth while, though for the moment, and mind you this can change at short notice, The Confidence Gap is my favourite in terms of readability and style of writing. The Happiness Trap has become a personal development classic and the book you should start with if you are interested in ACT.