Dry, Stark
Slaughterhouse-Five is written in the stylistic equivalent of a November landscape. If you're looking for flowery, check out some Byron. If you're looking for wartime realness, however, look no further.
As we discuss in our "Tone" section, there aren't too many adjectives floating around Slaughterhouse-Five. The novel's writing is minimalist and dry, and Vonnegut tends to write in short, declarative sentences. Each tiny section is dense with dialogue and action. For example, check out Chapter 3, Section 29 (and bear in mind, this is the whole section):
The war was nearly over. The locomotives began to move east in late December. The war would end in May. German prisons everywhere were absolutely full, and there was no longer any food for the prisoners to eat, and no longer any fuel to keep them warm. And yet—here came more prisoners. (3.29.1)
This brief passage tells us (a) when the events of this part of the book are taking place (late December) and (b) what waits in the immediate future (the end of the war). It also outlines the main problems that Billy Pilgrim faces as a POW during this part of the war: (a) there is no room for him in German prisons, (b) there is no food for him to eat, and (c) there is nothing to keep him warm.
Still, even amid all this straightforward, unadorned writing, there is an element of black humor. The book sets out the terrible conditions Billy will be living with, but it doesn't stop there. It adds the awful irony that, even though these conditions are so terrible, the war keeps making them worse by piling on more and more prisoners. Slaughterhouse-Five is filled with blunt, grim ironies like this one, which is why we describe its style as stark.