וְדִבְרֵ֣י רְחַבְעָ֗ם הָרִאשֹׁנִים֙ וְהָאֲ֣חַרוֹנִ֔ים הֲלֹא־הֵ֨ם כְּתוּבִ֜ים בְּדִבְרֵ֨י שְׁמַֽעְיָ֧ה הַנָּבִ֛יא וְעִדּ֥וֹ הַחֹזֶ֖ה לְהִתְיַחֵ֑שׂ וּמִלְחֲמ֧וֹת רְחַבְעָ֛ם וְיָרָבְעָ֖ם כָּל־הַיָּמִֽים׃
Now the acts of Rehoboam, first and last, are they not written in the histories of Shemaiah the prophet and of Iddo the seer, after the manner of genealogies? And there were wars between Rehoboam and Jeroboam continually.
Ibn Ezra
In the Book of the Wars of Adonoy. This was a separate Book, in which the Wars of Hashem on behalf of those who fear Him were written. Possibly, it existed in the days of Avraham, for many books were lost and are not extant anymore, such as the words of Nasan and Ido, the Divrei HaYamim of the Kings of Yisroel, the Songs of Shlomo and his Proverbs.