
This popular but slightly technical book is the text for a 3-part series of popular classes by Swami Sarvapriyananda on the Nectar of Supreme Knowledge. You can also see a lecture by the translator, Swami Sarvadevananda at this link.
The essential message is that the visible universe is a vast appearance without lasting reality. It is also Brahman, pure consciousness ever-existing in all forms and in its own majesty. That truth, beyond speech and mind, cannot be understood by the misleading imagination of the mind or expressed in language. The truth can only be known by a tranquil mind.
Human attempts to express that experience in words only gives rise to many different doctrines and dogmas.