The Transcendent Unity of Religion - Frithjof Schuon

https://archive.org/details/pdfy-lBasIMfuA724PVLO

The Underlying Religion: An Introduction to the Perennial Philosophy - Edited by Martin Lings & Clinton Minaar

https://archive.org/stream/MartinLings/Lings%2C%20Martin%20%5Beditor%5D%20-%20The%20Underlying%20Religion%2C%20An%20Introduction%20to%20the%20Perennial%20Philosophy%20%282007%29#page/n0/mode/2up