One modern or postmodern "memoir" that is short and that might appeal to eleventh graders is The Things They Carried, by Tim O'Brien. The book consists of many chapters that can be read in isolation, and it is difficult to determine how much of the work is "factual" and how much of it is "fictional." Indeed, O'Brien deliberately complicates any such distinctions. The topics the book explores are likely to fascinate many young readers, since much of the content deals with the Vietnam War and what it was actually like to take part in that conflict and to be a young person during that era.
One chapter of the book that particularly raises intriguing questions about the distinctions between reality and fiction is titled "How to Tell a True War Sstory." By the end of the chapter, we realize just how difficult telling such a story can be. Late in the story, the narrator comments,
How do you generalize?
War is hell, but that's not the half of it, because war is also mystery and terror and adventure and courage and discovery and holiness and pity and despair and longing and love. War is nasty; war is fun. War is thrilling; war is drudgery. War makes you a man; war makes you dead.
Even later in the same chapter, the narrator is even more succinct:
In war you lose your sense of the definite, hence your sense of truth itself, and therefore it's safe to say that in a true war story nothing is ever absolutely true.
The Things They Carried is a very thought-provoking novel -- thought-provoking not just about the issues already mentioned but also about many of the most serious issues in anyone's life. It is very well written and yet is at the same time easy to read. My prediction is that eleventh graders would be fascinated by it, but as most readers of the book at any age have been.
Please note that there is a short story titled "The Things They Carried" and that there is also a longer collection of related stories titled The Things They Carried. Most of the links below refer to the short story.
The Autobiography of Benjamin Franklin, Up From Slavery, Andersonville by John McElroy, Secret Knowledge by David Mamet, and 'Tis by Frank McCourt.