It's in the same book as Fiery Burst. It's called Storm Bolt, does damage in a 20 ft. line instead of fiery burst's ranged burst, reflex for half damage as you'd expect. Same damage and save DC calculations as Fiery Burst, though the prerequisite spell level is one higher, I think.
Personally, if you just want the best elemental reserve feat, storm bolt is NOT it. Twenty feet is often too close for comfort IME. If you're already a lightning mage, then it's worth just for the +1 CL, of course. The latter is the reason to get it, though. The best part of Fiery Burst is that no one can tell you caused it if you're smart about it. Plus...fire burns things. ^_^
My favorite is the acid one, though (again, same book -- Complete Mage).