Nostalgia and loss
The poetry suggests a deep sense of longing for the past, especially for the time that preceded the major losses that shape the life of the poet, who lost a child, we learn in "Majority." Nostalgia causes the associations of the poetry to constantly refer to this emotional bereavement, to suggest that it is not a pain that time assuages, but rather, it is a permanent wound that he carries through time that becomes a kind of sublime awareness of loss and pain. He experiences the loss as a filter that shapes his future experiences, and celebrates or regrets that in his poetry.
Time and age
The poetry is concerned with the passage of time, especially when he considers the age of people around him, or the way time affects his own life. He discusses the age his dead child might have been if they survived, and the collection is called Interrogations at Noon which is a suggestion that his own sense of middle-age is shaping the color of his poetry. The word "noon" helps to put these poems in the quickly fading day, so that the reader understands the gravitas of time in the poet's life.
Death
If time is an object of mystery in these poems, than death is the perplexing factor in it, because death has stopped his family from going down the path more often followed. He didn't get to enjoy raising his child, but instead, he watches other parents raising their children with a heightened awareness that the parents will die, and so will the children. He explores paradise lost in "The Lost Garden," and "Litany" is literally a death poem. The poetry is about death from start to finish.
