Fushigi Yuugi: The Story of the ShijinTenchishou
Fushigi Yuugi is generally translated as "Mysterious Play", and chronicles the adventures of Miaka and Yui inside the book known as the ShijinTenchishou. This is generally translated in English as "Universe of the Four Gods" although more literally it means "The Book of the Four Gods of Heaven and Earth".
There are eighteen volumes of manga, fifty two episodes of a Television series plus two OVAs and an Eikouden OVA based on the Gaiden novels of the same name. Drama CDs were also produced prior to the anime, featuring different seiyuu - but I don't have these yet s0 don't know how closely they mirror the manga.
The animated television series does follow the original story fairly closely up as far as volume thirteen although a few things are different. The later five volumes of manga are condensed into the second OVA, meaning that a lot of things are simplified or changed for ease of conveyance.
There are also thirteen Gaiden novels written as additional stories and background to the characters. They have never been translated into English and only Eikouden (much abridged) has been animated.
Watase-sensei's world is based loosely on a genuine ancient Chinese faith involving the stars. The names of the Seishi characters are therefore names derived from this source.