Dungeoneer's Survival Guide (Advanced Dungeons and Dragons)

Rating:
Summary: Extensions of the rules
Review: I can't say that this out-of-print book is important, necessary or even interesting for the AD&D player, but it does contain a relative wealth of rules and information about underground locales (water, cave-ins, etc.) and also presents a not-too-badly done version of the Underdark (the one prior to the existance of Menzoberranzan).

Rating:
Summary: Solid compendium of lore for major netherworld adventures
Review: This book will add amazing depth to your campaigns, by providing you with exhaustive information on caverns and the depths of the Underdark. Included are rules on the Underdark, swimming, climbing, bridges, falling, spelunking, fatigue, skills, combat underground, BattleSystem wars underground, cave-ins, mining, new equipment, and much more!