Oh and also high level instances and raids drop the "XXX Half of Armoursmithing: Volume Y"
There are 3 Armoursmithing Books you can get (Volume:1,2 & 3) and you need the top and bottom half of those books dropped in various different instances. These plans make the expensive "Enchant Thorium" Plate armour.
Once you made a book from combining both top and bottom halves of the same volume you can start the quest given from the book and hand it in, in Dire Maul North in the library.