This article is from the Anime FAQ, by Steve Pearl with numerous contributions by others.
The original RoLW was a pseudo D&D campaign, based on a much simplified verison of its rules. As its publicity grew, the gaming group, Group SNE, began to explain some of the rules at the back of their novels and RPG replays. The first attempt at publishing a complete system was the printing of the RoLW Companion in October 1989. The success it enjoyed prompted the release of volume two, which came out in June 1991, soon after the series has been animated. These books may be available at Kinokuniya or other major Japanese bookstores.
-From a post by Chadwick Sheeta "the Elf"