Jamieson, Fausset and Brown on Jeremiah 36:2

2. roll of a book--a book formed of prepared skins made into a roll. Compare "volume of the book," that is, the Pentateuch (Psa 40:7). It does not follow that his prophecies were not before committed to writing; what is implied is, they were now written together in one volume, so as to be read continuously to the Jews in the temple.

against ... nations--(Jr 25:15, &c.).

from ... days of Josiah--(Jr 25:3). From Josiah's thirteenth year (Jr 1:2).