The answer to your question depends entirely on how you define "book."
If you mean printed book, our oldest item is from the 1460s (I can track down the exact date, but I'll need to do some comparisons). If you mean oldest complete book (as in codex format), that would be the Speculum sanctorale by Bernardus Gui, a complete manuscript codex from ca. 1370. If you mean item from a book, as in a fragmet, that would be a fragment of St. Augustine's Tractates on the Gospel of John, ca. 1075. If you mean our oldest form of written document, that would be our set of Babylonian cuneiform tablets, the oldest of which dates to approximately 2500 BCE.
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