Nihonkoki History Book (日本後紀)

Nihonkoki History Book is a collection of histories created by imperial command; it was compiled in the early Heian period after Shoku-Nihongi (Continuation of Chronicles of Japan), being the third volume of Rikkokushi (Six National Histories). Completed in 840, it contains records of events that occurred over a period of 42 years, from 792 to 833. FUJIWARA no Otsugu and others compiled it. Written in Chinese, it was arranged in chronological order and comprised 40-volumes, 10 of which are extant.

Compilation
According to the preface, in 819 Emperor Saga ordered FUJIWARA no Fuyutsugu, FUJIWARA no Otsugu, FUJIWARA no Sadatsugu and YOSHIMINE no Yasuyo to compile the history book. Because all of them except Otsugu died before the work could be completed, Emperor Junna ordered KIYOHARA no Natsuno, Naoyo-o, FUJIWARA no Yoshino, ONO no Minemori, SAKANOUE no Imatsugu and SHIMADA no Kiyota to continue the project. During the reign of Emperor Ninmyo, more people such as FUJIWARA no Otsugu, MINAMOTO no Tokiwa, FUJIWARA no Yoshino, FUJIWARA no Yoshifusa and ASANO no Katori were called to carry out the work. Subsequently, FURU no Takaniwa and YAMATA no Furutsugu joined the compilation, and at last the book was completed on January 5, 841. It took 21 years to compile the book, but only FUJIWARA no Otsugu had been involved in the whole project over the three generations of emperors.

Contents
Following "Shoku-Nihongi," the book contains records of the reigns of emperors Kanmu, Heizei, Saga and Junna.

Until about the early fifteenth century, all 40 volumes of Nihonkoki History Book were in existence, but eventually certain volumes were dispersed and lost; however, Yukinori INAYAMA, one of the disciples of Hokinoichi HANAWA, found 10 volumes of the manuscript at the residence of the Sanjonishi family and published them for the first time (There is an opposing view that the published volumes were based on Norimitsu/Motomitsu YANAGIHARA's recension.). The 10 existing volumes are numbers 5, 8, 12, 13 (Kanmu), 14, 17 (Heizei), 20, 21, 22 and 24 (Saga); all the volumes about Emperor Junna are missing. It is to some extent possible to restore the text by gathering quotations from "Nihongi ryaku (The Abbreviated History of Japan)," extracts from Rikkokushi and "Ruiju kokushi (History Book)," a categorized Rikkokushi. "Nihon isshi History Book" and "Unknown or Lost Writings of the Nihonkoki History Book" are the collections of the lost texts of Nihonkoki History Book, and with them as a basis the following books were published: "Japanese Historical Records Commentary," from Shueisha Inc., and "Academic Paperback Library," from Kodansha Ltd.

It is said that the contents of the book generally reflected the opinions of FUJIWARA no Otsugu. As was the case with "Shoku-Nihongi," short biographies called koden were added to the obituaries of the emperors and retainers of the Imperial Court, which was the same approach seen in the subsequent history books. However, the koden in this book was neither mere praise nor criticism, but was instead full of interesting descriptions interwoven with unique comments and impressions, and because of its critical spirit the book was unusual among Rikkokushi.

Also, it is known that compared with the volumes compiled before and after the book, it contained more Waka/Yamatouta poems than the other volumes of Rikkokushi.