What can Slaughterhouse-Five be compared to in an essay?

Catch 22 has been suggested already, but that is the first novel that comes to mind and probably the most direct and obviously compatible connection to Slaughterhouse Five.

An accessible, albeit non-fiction piece of literature you could also use would be Marcus Aurelius' Meditations, the definitive work on the philosophy of stoicism.

What can Slaughterhouse-Five be compared to in an essay?

Let me suggest a psychological dimension: Since Billy Pilgrim is "unstuck in time" you could compare his experience of the world with other works in which there are altered perceptions of time, identity, and memory. Some movies like "Eternal Sunshine of the Spotless Mind" and "Memento" come to mind. As for written works, perhaps something by Philip K. Dick like "Ubik" would fit the bill or, better yet, "The Man in the High Tower." For that matter, Vonnegut himself played with some of these themes in his early work "The Sirens of Titan" and in "Mother Night." 

What can Slaughterhouse-Five be compared to in an essay?

You can do a comparison on a couple of different topics:

Historical Angle - You could look at other literature depicting World War II or the effects of this war - Elie Wiesel's Night, The Diary of Anne Frank

Stylistic Angle - Vonnegut uses a great deal of black humor to make social comments - Some call him a modern - day Mark Twain, so any of Mark Twain's works - particularly his satire, like "The Lowest Animal" would be appropriate. 

Science - Fiction Angle - Vonnegut uses science fiction as a motif, so it may be useful to look at other science - fiction works, particularly those with strong social commentary - Ray Bradbury, Ursula LeGuin.

What can Slaughterhouse-Five be compared to in an essay?

"Slaughterhouse Five" can be compared to any Modern piece of literature insofar as its structure is concerned. The novel is fragmented. It skips from the past to the future. It alternates points of view. This style is mimetic of many novels written shortly after WWI. You could compare the structure of "Slaughterhouse Five" to any novel by Evelyn Waugh, Graham Greene, and especially James Joyce.

What can Slaughterhouse-Five be compared to in an essay?

Slaughterhouse Five can be compared to Huck Finn -- people have called Vonnegut a "modern day Mark Twain." 

You could also compare it to Catch-22 by Joseph Heller, which is another darkly humorous war novel with a nonlinear plot.

If you want to compare books with non-linear plot lines, you could talk more about structure of the novels -- some other nonlinear novels are Wuthering Heights by Bronte, Ulysses by Joyce, or Trainspotting by Welsh (there are many more as well!)

Homer's Illiad or Odyssey could be compared to Billy Pilgrim's journey.

If you want to stick to the war theme, you could compare it to any other war novel, especially those set during the same time period. 

I hope this gives you a few ideas.