Adam Clarke on 2 Chronicles 35:25

Behold, they are written in the lamentations - The Hebrews had poetical compositions for all great and important events, military songs, songs of triumph, epithalamia or marriage odes, funeral elegies, etc. Several of these are preserved in different parts of the historical books of Scripture, and these were generally made by prophets or inspired men. That composed on the tragical end of this good king by Jeremiah is now lost. The Targum says, "Jeremiah bewailed Josiah with a great lamentation; and all the chiefs and matrons sing these lamentations concerning Josiah to the present day, and it was a statute in Israel annually to bewail Josiah. Behold, these are written in the book of Lamentations, which Baruch wrote down from the mouth of Jeremiah.