I am not sure if that is right... In my copy of the Infernalist it states that you have to know 5+ Calling and/or Shadow spells. If I am right - and hopefully so - you can take Infernalist with just 5 spells of one or the other discipline. In my case I decided to take 5 spells of the calling spells per extra discipline feat. Just the minimum token amount to qualify for the Infernalist.
In that case a normally calm scholarly war wizard, but his demonic side gained through a little trip to hell is driving him slowly into unknown territory.
I took Deral's comment to indicate that Spellbound's version of Spell Library didn't have a school limitation. If that's not the case, sure, Extra Discipline is the only way to go. And you're right, it is calling and/or shadow.
Nope, sorry for the misinformation, it's Extra Discipline you'd need