a written document: a scroll, book, or official paper set down to be read
In the biblical world a biblion was almost always a scroll, a length of papyrus or parchment written on and rolled up, not a bound volume with pages. That physical picture matters: Jesus is handed the scroll of Isaiah, unrolls it, reads, then rolls it back up (Luke 4). Whatever its contents, the word names a piece of writing meant to be read, from a prophet's book to a personal letter to a legal paper.
The range runs in three directions that share this one core. Most often it is a book or scroll in the ordinary sense: a sacred text, a written record, the volume John is told to write. In a fixed legal phrase it narrows to an official certificate, above all the certificate of divorce a man handed his wife. And in Revelation it repeatedly names a heavenly book, the record in which God keeps names or deeds, opened on the day of judgment.
The word is a smaller form of the plainer word for book, and by New Testament times that shrinking force had mostly worn off, so biblion simply means book or scroll with no stress on size. When a writer wants to stress that a scroll is small, he reaches for a different word (the little scroll John eats in Revelation 10).