John Muir, My First Summer in the Sierra
Muir was a deeply spiritual man, though I understand from reading this article here, that he may have abandoned conventional religious thought as he got older. It seems to me that being in nature and wondering about its mystery may have been his true religion.
I am currently reading the above mentioned book and have put it down many times after not having gotten very far. Insight I think is often revealed with persistent and patient effort but that just isn't easy. What you might get out of something isn't always immediately obvious and can take time to understand. Muir's writing seemed ponderous to me at first until I began to appreciate how much he must have been in awe of what he saw around him. His descriptions of the natural world are lyrical and eloquent, succinct yet expansive. I am learning to read his words in small bites in order to appreciate them more. The above book is a good introduction to his thoughts.