Book of Gad the Seer
The Book of Gad the Seer (Hebrew: דברי גד החזה, romanized: diḇrê Gāḏ ha-ḥōzeh) is a presumed lost text, supposed to have been written by the biblical prophet Gad, which is mentioned at 1 Chronicles (1 Chronicles 29:29). The passage reads: "Now the acts of David the king, first and last, behold, they are written in the book of Samuel the seer, and in the book of Nathan the prophet, and in the book of Gad the seer."
Some traditional Rabbinic commentaries understood this to be a reference to the books of I and II Samuel which were started by Samuel himself and completed by Nathan the Prophet and Gad the Seer.