Information on how to do self inquiry can be obtained from many sources. However in order for self inquiry to be effective it is necessary to go beyond the mind. It is almost impossible to "get" the mind with the mind (intellectually).
Yogani's book covers this aspect of self inquiry, emphasizing the importance of having a daily meditation practice in place that will help transcend the mind and increase inner silence (awaken the witness in you). Once the witness is awake, self inquiry becomes a way of life rather than something you have to work at to "get". He explains how self inquiry is not about running away from the world, but to be completely immersed in your world, filled with love and compassion, without being mentally and emotionally attached to it in a clingy way.
This book covers the five stages of mind on the way to spiritual maturity: Pre-Witnessing, Witnessing, Discrimination, Dispassion, and Merging of Subject and Object in Outpouring Divine Love. The stage of mind will determine what kind of self inquiry will work the best. Taking on more than you can handle will definitely slow down progress. Self Inquiry works best when the witness is present. In this book, Yogani calls self inquiry with the witness already established (through meditation), "relational self inquiry."
Two main kinds of self inquiry are covered in this book. There is self inquiry that arises in our every day life. We can inquire into things that worry us or bother us or make us unhappy, things that are considered "mental baggage" and how to free ourselves from this. The other kind of self inquiry is for the ultimate truth. These are questions that arise in us like "Who am I?" and "Who is doing this?" The answers to these questions are found by letting go into our silent witness.
He points out the pitfalls the mind can fall into along the way on a path of self inquiry, and how to avoid them -- over analyzing, being deceived by the feeling of having "arrived," and claiming life to be "unreal" while retreating from responsibilities. He also shows how self inquiry fits into the 8 limbs of yoga, including covering the intimate and powerful relationship between samyama (an advanced yoga practice) and self inquiry.
Most of what Yogani has written has been my experience too, and I definitely recommend this book for people who are thinking of starting out with self inquiry. This will give you a good set of guidelines to follow as you move along. If you are into self inquiry already, it will be a good book to go through for a reality check.
It does not matter whether one follows a down-to-earth approach to self inquiry like Byron Katie or Eckhart Tolle, or an "ultimate truth" approach like Ramana Maharishi or Nisagardatta. This book is very useful for understanding and effectively utilizing any system of self inquiry.