Lines 32-44 Summary

Get out the microscope, because we’re going through this poem line-by-line.

In my darkest night,
when the moon was covered
and I roamed through wreckage,

Here, the speaker relates his toughest times in life to being lost on a pitch-black night.
Dark, nature imagery, "the moon was covered," is used in line 33 to show the speaker in need of a guiding light to help him navigate out of "wreckage," or treacherous terrain of life.

a nimbus-clouded voice
directed me:
"Live in the layers,
not on the litter."

Holy clouds, the poem's back to being surreal with a voice speaking from the clouds.
In connection with the previous lines, the speaker is able to endure his "darkest night" by following a guiding voice, or principle of being (see lines 4-6), who speaks directly only in lines 37 and 38: "Live in the layers, / not on the litter."
Lines 37 and 38 contain a quotation, so the voice there is definitely outside of the speaker's mind or consciousness. This voice also appears to be coming from someone pretty powerful (or even a big man upstairs).
In line 35, don't let "nimbus-" throw you. It means like a gray rain cloud, or atmospheric. Kunitz wants to make it clear that this cloud voice has some serious presence.
Lines 37 and 38 are incredibly sonic lines with alliteration, rhythm, and slant rhyme happening. They are definitely designed to grab our attention. See "Sound Check" for elaboration on these techniques.
Up to this point, we've had some clues about "the layers" meaning something in the vein of a deep, conscious exploration of life. But what about "the litter"? Think of it as surface debris, like cups and discarded nachos we trip over after an epic concert.
Similarly, superficial and fleeting things in life can stress us out, get in our way, and distract us from what we really care about, or what really matters in life. Let's do the math: Layers = life is centered. Litter = life is a mess. (And the moral of this story? Recycle!)

Though I lack the art
to decipher it,

Kunitz is having a metapoetic moment here, that is—he's acknowledging the limitations of his art form, poetry.
The speaker is unable to use language to accurately convey the future (because it hasn't happened yet) in the same way that he was able to describe the past.

no doubt the next chapter
in my book of transformations
is already written.

Art and writing are directly addressed again, but this time with a twist. The speaker uses a book metaphor, "next chapter," to stand for the next phase in his life.
The word "transformations" is a doozy and represents the big changes that will continue to happen to the speaker (like a cocoon to a butterfly, to a huge butterfly, to a butterfly with a wizard beard, or something.)
Also, line 43, "is already written," implies a conflation of time. This means that the past, present, and future are presented in a jumbled, non-linear fashion. Within the poem, this could suggest that life, itself, isn't linear (see lines 1-3). Or, it could mean that other forces floating around out there have an influence on the speaker's life beyond his comprehension (see lines 35-38).
Or, it could totally mean both.

I am not done with my changes.

This dog has not had his day. Kunitz concludes with a very succinct statement, hitting home the idea that life is about continuation and renewal, not endings or death.
An enduring quality of human existence (or consciousness) is presented outside of space and time limitations. The speaker, even when facing physical death or old age, only sees infinite possibilities for growth ahead of him.
It might be a little strange, but it sure is an inspiring and optimistic perspective.
All together now: Forever young, I wanna be, forever young.