Similar to rods, spell staves had a set list of spells that they could cast. However, instead of casting from their own mana reservoir, they cast from the player's. Spells cast required only (MP/2 + 1) MP to cast, and incurred no hunger cost. Needless to say, these were powerful items, granting the player access to cheap, hungerless spells. Moreover, using these staves still trained spell skills, as the Evocations skill did not even exist yet.
These were reworked into rods at some point before DCSS, and given their own mana reservoir in the very first version of DCSS (0.1).