Overall I would have to say this is an excellent book that combines principles of Psychology and Tibetan Buddhism with the focus on practices of compassion.
There are many techniques in this book that can help to cultivate compassion as well as insights and stories that help to illustrate his various ideas. For this I gave the book 5 stars.
That being said, There are times that the author seems to slide down a "slippery slope" into the worst sort of New Age thinking. For example, he seems to think that most difficulties in relationships are caused by one's own projections.
While one's inaccurate projections may be a part of many people's relationship dynamics, I think it is unrealistic to always assume that one's difficulties in relationships are based on this idea. This is an oversimplification of the reality of many situations and a poor attempt to fit everything into a nice little box.
For example, I have many friends who were born into or married into families to whom conflict and negativity is the norm (though in the case of those who married into it their spouses may not indulge in this behavior but rather their spouse's families) and they became unwilling participants often as targets in this web of negativity. One may have compassion for why their aggressors developed such negative behavior, but to blame the recipient of this aggression by saying that somehow it is a manifestation of their own projections is absurd and not helpful. All this does is lead to unwarranted confusion and guilt in the person being victimized and traps them in a vicious cycle.
Sometimes in spite of their best efforts there is nothing they can do but "draw a line in the sand" and tell their aggressors that if they cross the line that they are no longer welcome. This can be the most compassionate thing one can do because that can stimulate the aggressor to look at themselves in the mirror and try to help themselves. These aggressors, if given free reign (like petty dictators), typically just get worse rather than better unless confronted firmly. It's almost like a power hungry person craving more power.
All that being said....
Overall, the material presented does offer many useful insights from ancient Tibetan Buddhist practices. If the Dalai Lama is an example of the fruits of these practices then one would have to be impressed by their potential.
Some of the practices described here are also described in the book "Awakening The Buddha Within" by Lama Surya Das (aka Jeffrey Miller) which is a fabulous book. While that is not a psychology book per se, Buddhism is very much psychological in nature in and of itself.
My favorite book that contains insights of psychology and spirituality is "Yoga and The Quest For The True Self" by Stephen Cope (also a psychotherapist as well as scholar in residence at the Kripalu Center in Massachussetts). I found that I was able to relate better to the material in that book overall than the Ladner book. Stephen Cope's book is also not only incredibly informative but is written in a style that makes it a joy to read (I am currently reading it for the 3rd time and I know it is a book that will be an ongoing part of my spiritual life).
Still, if you are interested in the meeting of psychology with spirituality, I would recommend reading and trying to incorporate many of the ideas "The Lost Art Of Compassion" into your life. It is a very readable book in terms of it's language (you won't need to read it with an unabridged dictionary by your side) and the author does seem very knowledgeable and sincere.