וְדִבְרֵי֙ דָּוִ֣יד הַמֶּ֔לֶךְ הָרִאשֹׁנִ֖ים וְהָאֲחרֹנִ֑ים הִנָּ֣ם כְּתוּבִ֗ים עַל־דִּבְרֵי֙ שְׁמוּאֵ֣ל הָרֹאֶ֔ה וְעַל־דִּבְרֵי֙ נָתָ֣ן הַנָּבִ֔יא וְעַל־דִּבְרֵ֖י גָּ֥ד הַחֹזֶֽה׃
Now the acts of David the king, first and last, behold, they are written in the words of Samuel the seer, and in the words of Nathan the prophet, and in the words of Gad the seer;
Ibn Ezra
THE Lord is One, and His divine inspiration always the same. The prophecies, however, vary according to the receptive faculty of each prophet, which is not always the same. Some prophets received the inspiration only in a vision by night. Such were, e.g., Abraham (Gen. 15:1), Gad the Seer (1 Chr. 29:29), and Isaiah (1:1).
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.