The contenders:
* The clay tablets from Mesopotamia (c. 3000 BCE): These tablets, inscribed with cuneiform writing, contain records, stories, and legal documents. They are often considered the earliest form of writing, and arguably the oldest "books" if we include any inscribed object.
* The Dead Sea Scrolls (c. 3rd century BCE - 1st century CE): These scrolls contain ancient Hebrew texts, including parts of the Bible. While not the earliest form of writing, their age and content make them incredibly significant.
* The Book of the Dead (c. 1550 BCE - 500 BCE): This collection of ancient Egyptian funerary texts was written on papyrus scrolls. It contains spells and instructions for the afterlife, and its importance in Egyptian culture solidifies its place among the oldest "books."
* The Diamond Sutra (c. 868 CE): This Buddhist text is the oldest printed book known, printed using woodblock printing in China.
So, which is the oldest?
It depends on your definition of "book." If you're looking for the earliest form of writing, the Mesopotamian clay tablets are the oldest. However, if you're looking for the oldest surviving printed text, the Diamond Sutra takes the title. Ultimately, the "oldest book" is a subjective determination.