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.
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 "...
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