As you can read in the introduction, this book is not intended for the beginning meditation practitioner.
It is based on the meditation methods found in the Commentary on the Visuddhimagga.
This includes a detailed analysis of mental processes and matter according to the categories of the Abhidhamma.
It is therefore advised to first learn more about the Visuddhimagga and Abhidhamma because otherwise it may be difficult to get a clear picture of the meditation practice described.