Not a bad book, although the title is a bit of a misnomer. It should really be called "The Shiva Process" or "Self-inquiry as a psychological tool", rather than just "Self-inquiry". People familiar with the term would normally equate it with the practice of Atma Vichara or that used in Advaita Vedanta, Nonduality etc.
In this book, the practice is used in a specific manner, along with psychological techniques and group work, more as a 'self-help' tool or a psychological tool in analysing states of mind, and moving from unhelpful thoughts to more helpful thoughts and states of mind.
The book contains some concepts from Kashmiri Shaivism, although in a very simplified manner. Likewise some concepts are borrowed from Gurdjieff's work (again simplified), which the author seems to have had some experience with.
The actual processes detailed are interesting, such as classing thoughts/beliefs into A,B, and G statements- which relate to the content of beliefs. Some practices are given around sensing energy in the body, chakras etc., and also on group work guidelines such as giving feedback.
It might work for some, or be of interest to those interested in these sorts of therapeutic/psychological processes, however, as for "self-inquiry" proper (in a nondual or ultimate sense), only about one page actually is relevant out of several hundred. There are a few ideas IMO that probably need to be straightened out as well- 1) the idea of no-thought isn't central or even stressed in self-inquiry, 2) watching thoughts and practicing a 'witness' consciousness isn't self-inquiry (since there's still someone attempting to be a witness, 3) self-inquiry is completely compatible with being 'in the world', and doesn't need some additional practices added in order to bring it into 'the world' or our language system, or 'back into the body'.
There's more that could be said, however, ultimately the book IS about the author's own process/system, and this needs to be borne in mind while reading and digesting what is said about practices in other traditions and 'self-inquiry'.