According to
www.otyaku.com, the following is the publishing order of 12K novels...
Masyou no Ko (魔性の子) "The Demonic Child"
Taiki's story in modern Japan
Tsuki no Kage, Kage no Umi (月の影 影の海)
"Shadow of the Moon, Sea of the Shadows"
Youko becomes Empress of Kei
Kaze no Umi, Meikyuu no Kishi (風の海 迷宮の岸)
"Sea of the Wind, Shore of the Labyrinth"
Taiki chooses the Emperor of Tai
Higashi no Wadatsumi, Nishi no Soukai (東の海神 西の滄海)
"Sea God in the East, Vast Sea in the West"
Emperor of En suppresses rebellion
Kaze no Banri, Reimei no Sora (風の万里 黎明の空)
"Thousand Miles of Wind, Sky of the Dawn"
Youko, Suzu and Syoukei free Wa Province in Kei
Tonan no Tsubasa (図南の翼) "The Aspired Wings"
Syusyou becomes Empress of Kyou
Tasogare no Kishi, Akatsuki no Sora (黄昏の岸 暁の天)
"Shore in Twilight, Sky at Daybreak"
Risai meets Youko to request help in summoning Taiki
Kasyo no Yume (華胥の幽夢) "The Dream of Prosperity"
Various short stories: Tai, Hou, Kei, Sai, Sou