If we want to get an answer to the question of which is the oldest book in the world, we must first define the concept of a book. By common convention, a book is a collection of written or printed pages bound together and provided with a cover. Due to this definition, for example, texts preserved on stone tablets or papyrus or metal cylinders cannot be considered books.
So we would be done with the definition, but then which is the oldest book in the world? Well, speaking of a printed book, the Guttenberg Bible would give itself as an answer, but with its 550 years, it is considered almost a baby next to the Diamond Sutra containing Buddhist sacred texts, whose age is estimated at about 1145 years.