A potter's field is where the poor or unnamed are buried, and the poem begins there. The speaker imagines "a thin book of days" as the thing the dead are buried in, almost like a shroud or a grave. It's a quiet, sad image: a life reduced to a slim record of days, with no ceremony and little to show for it.
The second stanza turns outward. "Elsewhere the wall" suggests a border wall or some other large barrier, though the poem never says exactly which. The line "no one able to pay for it" is pointed. Money is scarce when it comes to burying the poor, yet somewhere else a huge, expensive project is being planned or argued over. The poem doesn't explain this, which makes the contrast sharper.
The speaker's feeling seems to be restrained anger mixed with sorrow. Nothing is shouted. The short lines, the lack of punctuation and the plain wording give the poem a cold, bleak mood, as if the facts are enough to make the point.