The thesis of the book, and its a standalone book, is that there are common threads among religions and our lived history, cross-culturally and across time -- while it may appear that the focus is on "spirituality", Huxley goes beyond and makes some astute commentary on the history of our civilisation on-top of religious teachings, paired with human psychology, and emphasises on certain core tenets that he believes are "perennial" and that which exists at a higher plane which is what we must strive to achieve (which he refers to the divine plane, where the ultimate being "God" exists).

Here's how he open the book, by defining "The Perennial Philosophy":

"PHILOSOPHIA PERENNIS-the phrase was coined by Leibniz; but the thing--the metaphysic that recognizes a divine Reality substantial to the world of things and lives and minds; the psychology that finds in the soul something similar to, or even identical with, divine Reality; the ethic that places man's final end in the knowledge of the immanent and transcendent Ground of all being the thing is immemorial and universal."

If you feel curious -- I'd recommend reading a couple of chapters to see how comfortable you feel with his writing. His sentences are much more dense compared to his novels. Overall, this book has a solid thesis and leaves you with much to think about (and might even prompt you to change your mind on certain viewpoints)

:)