I already have firesoul and want to get a new one
or should I buy darktoad instead of celestial powers?
depends on your loadout, if you have no regen and decent armor along with a decent health pool you could buy dark toad. dark toad ain't the best damage spell but is used by alot of players even in late game to recover health.
if you want to go for pure damage buy celestial, but you could also save a little more and buy the blade of the throne which deals more damage in it's caster form then celestial does.
you could also buy the greater storm spellbook if you know how to dodge attacks and want to play risky.
Another option which is free yet not the easiest to pull off is to farm all the gardening seeds off enemies and unlocking the nature's wraith spellbook, as that spellbook has both healing properties aswell as maintaining some decent damage and movement prevention, be warned though this is no easy task and the healing is limited and can only be used every 26 seconds unlike the dark toad whose attacks have life steal.
Celestial Powers is a good spellbook, but there are better options. I would definitely buy it though, because Fantastic Beamstorm is very helpful in the Nightmare, for hitting through walls. The charged attack is also pretty good for headbaiting/fighting trapped enemies.
What do you think?