This is a scriptural commentary of Lahiri Mahasaya on The Upanishads.
If we scan the word Upanisad, we see that Upa means sitting and nisad means near. Thus, the very word Upanisad specifies personal relationship: the Guru-param-para, Master to Disciple learning from the living lips of a Guru.
In ancient times, the disciples sat near the Guru to learn the spiritual discipline from the living lips of their Guru to realize the supreme Self. They practiced strictly in accordance with the instructor they received personally from the living lips of their Guru. This is a Kriya Yoga book intended to be read and practiced by everyone, with/without initiation.
Please note that the Guru here means the Kutastha, i.e., the place between the eyebrows, also known as The Third Eye.
Every word uttered by a Yogi has a special meaning that is totally unintelligible to even the highly intellectual people. This book is written in such a way that everyone can follow it up while trading the path of Kriya. People think that they are very intelligent, but if they try to understand very seriously, they realize perfectly that nothing is happening according to their intellect.