Warlock Epic Spells : World of Warcraft General Discussions

I've been looking everywhere for this and I'm gonna die if I don't find out soon enough, lol. I'm aware warlocks gain the ability to summon Infernals and Doomguards at lvl 50 and 60 respectively. I've been given close to 10 different answers on how to get these spells. How and where do I find the ability to cast them? Also, on the Infernal spell, I've been told it has something to do with a book but where is this "book?"

The infernal has to be farmed from the warlocks in LBRS and is you will find it as a Grimoire of Inferno and it's BOP, so roll well.