The Book of the Grave
The Book of the Grave is a necromatic wizard's spellbook, most notable for containing the directions for a rare Penumbra cantrip, which is a lesser version of the more common Protective Penumbra spell.
It also contains the instructions for a magical ritual Breath of the Grave, which rots the caster's breath and sickens everyone within a 15-foot cone. After using the ritual, the caster receives penalties to stealth and diplomacy due to their unpleasant stench.
Although The Book of the Grave does not contain any illegal spells, the fact that it is written in the Necril language makes it illegal to own.