The Ancient Cornerstone Grimoire is an upgrade to the item called the Book of the Dead. Both items are out of hand items. You can find Ancient Cornerstone Grimoire in Onyxia (a dropdown item). Onyxia is the current broodmother of the Black Dragonflight in Azeroth, residing in Onyxia’s Lair. This is a 40 man instanced raid. The Book of the Dead is delivered by Balnazzar in Stratholme. This is a 55-60 dungeon in the Eastern Plaguelands, so you can do it only if you are level 70 or higher.
The Ancient Cornerstone Grimoire will give you 10 Stamina, 15 Intellect, and 11 Spirit. The Book of the Dead will give you 8 Stamina, 15 Intellect, and 10 Spirit. The book and grimoire have a 15-minute cooldown to summon a 60-second skeleton, which will fight for you. The skeleton itself is pretty strong and does good damage, it will also take away your aggro.
The skeletal summoning of the Book of the Dead and the Ancient Grimoire of the Keystone is the same. So the only advantage of the grimoire is the bonus stamina 2 and spirit 1. That said, you’ll also need to know that while the book has only a 2% drop rate, the grimoire has a 20% drop rate. If you’re 80 and have a friend or two, you can do it for the grimoire, but if you’re low level and just want to do it alone, you can go for the book and get the (almost) same item with less mach. effort on your part.
The two items come together when picked up, so if you want to get them and can’t do it yourself, you can always pay someone to help you out and accept that you’ll give the drop. A fair price will be 200-300 gold if it drops.