So some wizards could invent new spells, if they chose to.
There certainly won't be any definitive list of spells, a la D&D. That's not a slam on D&D (which I like and play regularly), but that sort of thing just isn't going to work for Dresden-style magic.
Dresden's magic system is pretty freeform. Evocation requires the player to describe what sort of effect they're going for, then decide how strong to make the effect (how many "shifts" of power they want, in FATE terminology). Then they make a roll on the appropriate skill and off they go. Thaumaturgy works in a similar way, but the player gets more time to make it work - it's a lot less likely for stuff to go wrong, but it takes a lot longer. The greater the number of shifts of effect the player wants, the tougher the roll. Reach too high and you risk inflicting consequences on yourself - or the neighboring architecture, bystanders, allies, etc. Reach too low and your opponent will shrug off the effect, burst through the wind wall, swat aside the hailstones you've conjured, or whatever.
Some characters (like wizards) get a certain number of "rote" spells, which are essentially canned spells that let the player skip the roll (like d20's "Take 10" - you assume a neutral roll and go with that value); it's quick and reliable, but not terribly flexible. Of course, a lot of times you don't need flexible. Harry's "Fuego!" is a good example of a rote.
