I know this can be important. When I hiked the JMT I brought along a book to read. Unfortunately it was Naomi Klein's Shock Doctrine which is a fantastic book but didnt have me excited about society when I got back.
Journey to the East by Herman Hesse
Its a book that is an allegory for our own spiritual journey. Not nature, but journey related so its relevant I think. The book is relatively short, a breeze to read, a personal favorite, and uplifting.
Quote: "For our goal was not only the East, or rather the East was not onlya country and something geographical, but it was the home and the youth of the soul, it was everywhere and nowhere, it was the union of all times."
The Prophet by Kahlil Gibran
About a person who's ship has arrived at port. He is about to leave his city forever and the people ask him to impart his truth. The book consists of his lessons spoken to the people.
Quote: "Life is indeed darkness save when there is urge,
And all urge is blind save when there is knowledge,
And all knowledge is vain save when there is work,
And all work is empty save when there is love."
On the Road by Jack Kerouac
Good chance he already read this but if not, its required reading.
Quote: "So in America when the sun goes down and I sit on the old broken-down river pier watching the long, long skies over New Jersey and sense all that raw land that rolls in one unbelievable huge bulge over to the West Coast, and all that road going, and all the people dreaming in the immensity of it... and tonight the stars'll be out, and don't you know that God is Pooh Bear?"