Ruiju Kokushi (類聚国史)
Ruiju Kokushi, completed and established in 892, is a history book edited by SUGAWARA no Michizane, which classified and recompiled the entries of six classical Japanese history texts compiled in chronological order, following the example of Leishu ("classified books") in China. In the period of the Tang Dynasty in China, it was widely done to excerpt necessary parts from old books to classify and recompile them for making prose and poetry or organizing knowledge, and such books were called Leishu. This book also follows the style of Leishu and therefore can be regarded as a Leishu.
Regarding the date of establishment, one theory is that since "Nihon Sandai Jitsuroku" (sixth of the six history texts) was completed in 901 and Michizane was exiled to Dazaifu just before that year, the excerpted part of the book was added at a later date. And the other theory is that since Michizane himself was involved in the compilation of the 'Sandai Jitsuroku' and had the draft before its completion, he added a few touches to it before his exile.
It originally came in 205 volumes consisting of 200 text books, two catalogs and three genealogical charts, but they were scattered and lost in the Onin War, and only 62 of them are now in existence. All entries are classified into 18 categories (Ruiju): gods of heaven and earth, the emperor, consort of the emperor, human affairs, events in the year, music, prize and party, dedication, political affairs, criminal law, organization of government, literature, rice fields, good omen, natural calamity, the teachings of Buddha, manners and customs, and strange customs. The lost part had five categories such as the crown prince, subjugation, and place-names according to Jun TAKADA, a professor at Kokugakuin University. It is worth noting that the book provides convenience for referring the precedents in the texts of the early Heian period in particular which are easy for searching, and unnecessary modifications to the text were all avoided true to the original text. It is also a valuable material to restore the "Nihon Koki" (one of the six history texts), many parts of which are now lost.