Fushigi Yuugi Gaiden

Over a series of five years, Megumi Nishizaki wrote thirteen Japanese, light novels based on Fushigi Yûgi. With illustrations by Yuu Watase, Fushigi Yûgi Gaiden primarily explores the lives the various Celestial Warriors before they are seen in the manga. The only two novels to be set after the manga, Eikō Den (Jōkan) and Eikō Den (Gekan), later became the basis for the third Fushigi Yûgi original video animation, Fushigi Yûgi Eikoden. Originally published by Shogakukan, none of the novels have been licensed for English release. (wiki)
Each book has around 10 chapters