What is "Sefer Divrei Shlomo"?
Radak: These are Seforim that he composed with his Chachmah. They are not found with us, only three - Mishlei, Shir ha'Shirim and Koheles 1 . It says Sefer (singular) because he composed a Sefer for each matter by itself.
Radak citing Shir ha'Shirim Rabah 1:10: He wrote Shir ha'Shirim in his childhood, Mishlei in his youth, and Koheles in his old age. Some say that close to his death Ru'ach ha'Kodesh came upon him, and he composed these three Seforim.
Why does it say here "v'Yeser Divrei Shlomo
Malbim (Divrei ha'Yamim II, 9:29): Nosar (or Yeser) implies that it remained automatically. There was no adamancy about his Chulin matters; what was not written here is in his chronicles. Divrei ha'Yamim discusses his Kodesh matters. Nish'ar is left over intentionally; they were not written in Divrei ha'Yamim, because Nevi'im made special Seforim from them.
