Rikkokushi (the Six National Histories) (六国史)

"Rikkokushi" represents six national historical books which were compiled by the Japanese nation under the ritsuryo codes in ancient times. They were mostly written in chronological order, partially taking on the factors of biographical historiography.

Summary
The compilation of history books was continued for a national project of Japan over the period ranging from the Asuka period to the early Heian period; six history books were produced, which are called Rikkokushi. Therefore, if people simply say Kokushi in Japan, they could mean Rikkokushi.

There is a record that "Tennoki" (a Record of the Emperors) and "National Record" were compiled before Nihonshoki (Chronicles of Japan), but these books don't exist. It is believed that although there was a plan to compile a national history book called 'Shinkokushi' (the New National History) after the Rikkokushi, it was not completed. After the Meiji Restoration, it was also planned to compile a history book covering the period after Rikkokushi, but it was not realized due to various reasons, and instead, Dai Nihon Shiryo (the Historical Materials of Japan) was produced.

List of the Rikkokushi
Nihonshoki - covering Japanese myths and the period until Emperor Jito (date of birth unknown-697) in thirty volumes (although a volume of genealogical tables seemed to be produced too, it was lost). It was completed in 720. Authored by Imperial Prince Toneri.

Shoku Nihongi (Chronicle of Japan Continued) - covering the period from Emperor Monmu to Emperor Kammu (697-791) in forty volumes. It was completed in 797. Authored by SUGANO no Mamichi, FUJIWARA no Tsugutada, and others.

Nihon Koki (Later Chronicle of Japan) - covering the period from Emperor Kammu to Emperor Junna (792-833) in forty volumes (however, thirty volumes were lost and only ten volumes exist.)
It was completed in 840. Authored by FUJIWARA no Fuyutsugu, FUJIWARA no Otsugu, and others.

Shoku Nihon Koki (Later Chronicle of Japan Continued) - covering the era of Emperor Ninmyo (833-850) in twenty volumes. It was completed in 869.
Authored by FUJIWARA no Yoshifusa, HARUZUMI no Yoshitada, and others

Nihon Montoku Tenno Jitsuroku (Veritable Records of Emperor Montoku of Japan) - covering the era of Emperor Montoku (850-858) in ten volumes. It was completed in 879. Authored by FUJIWARA no Mototsune, SUGAWARA no Koreyoshi, Yoshiomi SHIMADA, and others.

Nihon Sandai Jitsuroku (Veritable Records of Three Reigns of Japan) - covering the period from Emperor Seiwa to Emperor Kotoku (858-887) in fifty volumes. It was completed in 901. Authored by FUJIWARA no Tokihira, OKURA no Yoshiyuki, SUGAWARA no Michizane, and others.