I have been studying The Dharma and meditating for several years and have found this to be one of the most perfect guidebooks I have read. For those who don't know; Ajahn Chah is a famous monk in the Thai Forest Tradition of Therevada Buddhism. He was ordained as a Buddhist at the age of 20 and dedicated his entire life to following the teachings of the Buddha. He taught many well-known monks (such as Ajahn Brahm and Ajahn Sumedho) who have since gone on to set up successful Buddhist Centres across the world.
I purchased this book based on a recommendation from a fellow meditator whom I met on a retreat and have used it consistently alongside my practice for the last few months, digesting the teachings slowly and putting them in to practice. I must say that the teachings have really helped me to progress a lot both in learning how to work with the mind and with my understanding of the most important aspects of Dharma. The style of teaching here is remarkably simple yet extremely profound. Ajahn Chah expounds on all the common issues that serious meditators face when on the cushion such as understanding sensations, letting go, observing the mind, attaining mental balance, right attitudes to practice etc. Some familiarity with Buddhist teachings may be beneficial to make full use of the book, however explanations are given throughout of the most important aspects such as Precepts, Noble Truths, The Characteristics, Aggregates etc, all explained thoroughly. In particular I found the descriptions on handling mental defilements to be particularly brilliant as they cleared up a lot of confusion that was obstructing my progression.
If you are looking for a beginners manual I would probably look elsewhere first (Breath by Breath by Larry Rosenburg or Mindfulness Bliss & Beyond by Ajahn Brahm are excellent). But if you already have a good grounding in Buddhist teachings and maybe a few years practical experience under your belt then I couldn't recommend this book highly enough.