Both books are similar enough to describe together. Abyss was written in closer proximity to his cancer diagnosis. It is shorter, maybe more concise. Whereas, Zero, written with some distance from initial diagnosis and with some life in between, is a bit further reaching, maybe less immediately dark. 

Both are meditations of sorts confronting difficulty through poetry. Wiman’s cancer diagnosis is in the background, subtext and rarely specifically referenced but knowing it exists adds a grindstone of reality to his inquiry. It’s also a meditation through some (though definitely not all) specifically Christian poetry.