Only use I can think of for it is taking ignite for level 2/3 cheese
But then
Electrocute is far better than a bit of true damage
Plus the stats you get from the domination tree
You can take domination secondary and get the stats.
The problem with spellbook is that you usually only swap spells once MAYBE twice during a game. If it lowered the CD of summs a hair more and i think it would be closer to being considered a really strong key rune. The summ spell swapping isn't what really makes it strong, the reduction of summs CD is.
Like, some junglers are seeing success on it, but I still think it's terrible as a keystone. Literally the only thing I think would be useful is using a summoner, trading that summoner for smite, letting smite cooldown, and then changing the summoner back. So you can have the other summoner back for basically smites CD.
