The Ancient Cornerstone Grimoire is an upgrade of the item called Book of the Dead. The two items are off-hand items. You can find Ancient Cornerstone Grimoire on Onyxia (a drop down item). Onyxia is the current broodmother of the Black Dragonflight on Azeroth, residing in Onyxia’s Lair. This is a 40 man Instance raid. The Book of the Dead drop from Balnazzar in Stratholme. This is a 55-60 Dungeon in Eastern Plaguelands, so you can solo it if you are 70 level and up.
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 the grimoire have a 15 minute cool down on use for a summon of a 60 seconds skeleton, that will fight for you. The skeleton itself is quite strong and make a nice damage, he also will take agro off of you.
The skeleton summons from the Book of the Dead and from the Ancient Cornerstone Grimoire is the same. So the only advantage of the grimoire is the additional 2 stamina and 1 spirit. That said you will also need to know that while the book have only 2% drop rate, the grimoire have a 20% drop rate. If you are 80 and have one or two friends, you can make it for the grimoire, but if you are low level and you only want to do solo, you can go for the book and get the same (almost) item with mach less effort on your part.
The two items binds when picked up, so if you want to get them and you can’t do it yourself you can always pay someone to help you out and agrees that you will be giving the drop. A fair price will be 200 to 300 gold if it drops.