The allegory of Plato's Cave is meant to demonstrate that belief does not always equal knowledge, believers in only empirical knowledge are trapped in a cave of misunderstanding (the shadows, from the perspective of the prisoners, are real and an empirical truth for them).But how do we know if things as they appear in our relative reality are not a manifestation of how they appear when you're out of Plato's cave?
All things that appear in our relative reality are manifestations, coming and going, being created and destroyed, all without their own intrinsic nature. No theorizing needed, you can check this for yourself.
The rose we are contemplating in relative reality has no intrinsic nature of its own. There is no "rose-ness" there that you can find, its merely a manifestation of the causes and conditions that allow it to exist in that form - which is empty (Heart sutra).How do we know that the (relative) rose we are contemplating is not a form of expression of a rose in absolute reality?
I have not read the book you cited, but the taglines read "The Conquest of Illusion aims to teach those in the modern world - an existence dominated by physical things and technological achievement - of a consciousness which has and will always exist".... so this book would essentially be arguing the same thing we're pointing at here.
BUT having said all this, you're putting the car way in front of the horse! If you'd oblige me, I'd like to recommend that you stop reading any and all things related to spirituality, awakening, the nature of existence, non-duality, etc. The top of the mountain looks the same but there are many paths to get there...let's focus on this path of inquiry for now. Once we're on the other side of the gate I'd be more than happy to help you work through remaining questions...if you still have them!
Sound good?

