From the Introduction:
This guidebook describes essential information that applies to all Spellbook Games' Inferno products. The information is being published separately so that it is not repeated in every product. This guidebook includes the geography of Gehenna, rules pertaining to survival in Hell, and rules about healing and magic.
This product is intended for use by a Games Master as part of a role playing campaign, and contains some of the information necessary to guide a party of Adventurers as they travel through the environs of Hell. This guidebook does not stand alone. It does not describe any specific adventuring sites in Hell. Reference is made throughout, without additional explanation, to monsters, spells, prayers, and enchanted objects described in one or more of Spellbook Game's: Portal to Adventure RPG rules, the Codicil of Maladies, Inferno Bestiary, Inferno: Journey through Malebolge, and Inferno Treasury.
Gehenna and Inferno are locations for higher-level game play. It is suggested that Adventurers be at least L10 before visiting any part of Hell. Games Masters should not allow easy access to Gehenna, nor an easy retreat from it. The suggested prayer and spell restrictions are intended, in part, to prevent an easy retreat. Adventurers entering the Gates of Hell should not have magical means of escape readily available to them. It is intended that any mortal venturing inside Inferno be compelled to walk from the outer perimeter inward two hundred miles to Lucifer to make their escape, descending through ten Circles and a vertical mile. Each Circle presents a unique environment, ones that are often deadly to mortal life.