A
NetHack scroll that, while being useful, makes you
pine for
the real thing: a
spellbook of magic mapping.
When read and uncursed, the map of the current level is revealed. If it is cursed, only parts of the map are revealed. I guess you can't lose with this one...
Although most of the levels in the Dungeons of Doom are very straightforward, don't get too comfy. Gehennom's levels and the Elemental Planes are severely complicated -- personally, I wouldn't even venture in there without my trusty spellbook of magic mapping.