Arduin - the OTHER 3 little brown books! Heck, it even says ORIGINAL EDITION on the cover! This unusual 1981 boxed set from Grimoire Games has all three volumes of this self-contained hyper-weird OD&D mutation. It's all you need to be totally confused while having a blast. Magnifying glass recommended.
If you want to learn more about the contents of the Arduin GrimoireCLICK HERE to read Jeff Rients' excellent series of review posts.