In Slaughterhouse-Five, can Billy Pilgrim be said to be delusional, with strange events stemming from his own mind instead of reality?
In Slaughterhouse-Five, one possible interpretation of Billy Pilgrim's journey is that he has become delusional or insane from his wartime experiences. One clue to this interpretation comes from the very first lines of the book:
All this happened, more or less. The war parts, anyway, are pretty much true.
The bolded lines show that even Billy himself is unsure of what is real and what is not; the only thing he knows for sure is that his war experiences were real. The other things may happen only in his own mind as a reaction to his trauma; it is shown later that he is still suffering effects from the war:
Billy saw service with the infantry in Europe, and was taken prisoner by the Germans. After his honorable discharge from the Army in 1945, Billy again enrolled in the Ilium School of Optometry. During his senior year there, he became engaged to the daughter of the founder and owner of the school, and then suffered a mild nervous collapse.
(Vonnegut, Slaughterhouse-Five, Google Books)
Again, the bolded lines show a history of trauma that has clearly affected his mind; while he is not shown to be explicitly insane -- at least compared to the rest of the world -- he is suffering from Post-Traumatic Stress Disorder, at the very least, and cannot be said to be a completely accurate narrator. His honorable discharge was likely for stress-related reasons, not for physical injury, and his later collapse shows that he has not managed to settle his own mind.