In this case the poem and the song seem very different to me. The poem is very simple and 
straightforward--a series of four quatrains that each makes a simple statement about their 
love. By using the first verse as a refrain that repeats several times the song takes on a 
much borader and wistful feel. In that sense this song is less a colaboration and more of 
a reworking of the Teasdale poem into something that is more distinctly mine.

THE MYSTERY
Words by Sara Teasdale, music by James E. Nelson

Your eyes drink of me,
Love makes them shine,
Your eyes that lean
So close to mine.

We have long been lovers,
We know the range
Of each other's moods
And how they change.

But when we look
At each other so
Then we feel
How little we know;

The spirit eludes us,
Timid and free --
Can I ever know you
Or you know me?