Unfortunately, it has to be noted that in referring to "esoteric knowledge", many authors lead their readers astray.
The very word "esoteric" means secret, that is to say concealed from the majority of people and requiring particular understanding due to its profound meaning and the impossibility of expressing it in words.
But if authors shout about this esoteric knowledge from the rooftops and regularly mention it in their books, then the knowledge in question is no longer "esoteric", but rather "exoteric" (not a secret, intended for the uninitiated) – and the two concepts should not be confused.