Wrote in a book - Whether this book contained any more than is recorded in this place we do not know; probably it contained no more than what is found in Jeremiah 51:62-64. A book, ספר sepher, signifies, in Hebrew, any writing, great or small.
Historical appendix. In his fourth year Zedekiah journeyed to Babylon either to obtain some favor from Nebuchadnezzar, or because he was summoned to be present on some state occasion. Jeremiah took the opportunity of sending to the exiles at Babylon this prophecy.
Jeremiah 51:59
Seraiah - Brother to Baruch.
A quiet prince - literally, “prince of the resting place, i. e., quartermaster.” It was his business to ride forward each day, and select the place where the king would halt and pass the night.
Jeremiah 51:60
In a book - literally, in one book, on one scroll of parchment.
Jeremiah 51:61
And shalt see, and shalt read - Or, then see that thou read etc.
Jeremiah 51:62-64
The sinking of the scroll was not for the purpose of destroying it, but was a symbolic act (compare the marginal reference); and the binding of a stone to it signified the certainty of the hasty ruin of the city.
Jeremiah 51:64
Thus far - Whoever added Jeremiah 52, evidently felt it his duty to point out that it was not written by Jeremiah.