Here is the full text of the letter:
I write what shall be my last appeal to go unanswered, one way or the other.
I feel a prisoner, as on an island, with no jailor, no human soul for commune-- only my one mind, examing itself, endlessly, endlessly,
searching for relief.
In the years since transgression I have sought
no absolution, only bare forgiveness. In good faith I have removed
myself from all temptation, sacrificed to prove my commitment however
I can imagine.
Since Mother's passing I have yearned for nothing more than the acknowledgment of my own kin, to be treated as human again, to breathe
the air of human spirit once more. By grace even a wretch like me
could be saved, but I do not expect it. If not response is received, I
shall henceforth accept my sentence, and one day simply cease to be.
With a brother's love always,
The phrase in question, to breathe the air of human spirit once more, could be rewritten to say that the author wishes, once again, to do all of the things that make us human.
Friends, family, companionship, love, loss, joy, sorrow... it could be argued that it is the sum of these things that constitute the human spirit, and the writer wishes to experience them all once again. To savor them as you would savor the smell of a feast at a gathering of loved ones.
From the context of the letter, we can deduce that the author has done something which has rendered him an outcast, and he now feels less than human. The phrase "to breathe the air of...", in English, is commonly used as a metaphor for freedom, or escape, or of breaking free of a bond.
The prisoner yearned for nothing but to breathe the air of freedom once more.
To answer the second part of your question, could it be rephrased as "to drink the wine of human spirit", I would argue that it does not work as well, since the spirit is an intangible thing, like the wind, and is more appropriately represented by the breathing metaphor.
Oddly, and to complicate the matter even further, the phrase "to drink the milk of human kindness" (Macbeth, Shakespeare, 1605) is well established in English.
Hope that helps.