In the introductory essay "Cowboy Poetry Then and Now: An Overview" for the book Cowboy Poets and Cowboy Poetry, David Stanley asserted that this section of the poem,

My ceiling [is] the sky, my carpet the grass,
My music the lowing of herds as they pass;
My books are the brooks, my sermons the stones,
My parson's a wolf on a pulpit of bones.

takes its imagery "from Shakespeare's As You Like It, in which Duke Senior, living in banishment in the Forest of Arden, exclaims:

And this our life, exempt from public haunt,
Finds tongues in trees, books in the running brooks,
Sermons in stones, and good in everything:
I would not change it. (II, i, 15-18)"

Stanley cites this as evidence that McCandless was well-read, which I think is evident from the quality of the poem, though the one-line borrowing from Shakespeare need not indicate any direct familiarity. How many folks can quote famous lines from the Bard while thinking that Mercutio is a luxury sedan? The theme of the passage is exceedingly common in cowboy poetry, and the imagery may well have circulated orally prior to McCandless borrowing and expanding upon it in his poem.

According to Stanley, McCandless worked on the Crooked L Ranch in the Texas panhandle. I haven't encountered any other poems by him.