Now is the time to stack up on these sweet books, in cataclysm a lot less people will be able to farm them effectively in WotLK content and a lot people probably will be interested in the new Inscription system that will be implemented alongside with Cataclysm.
But if people are lacking glyphs they won't sell, which is why they need the books, 57 of them, each person.
You can't have enough of these books, if you don't want to bother with the glyphs, bug the ones crafting them with your ridiculous prices on the books.
Don't forget to stock up on herbs, any kind of herbs, vanilla, TBC, WotLK, stack them all up, they will rise in value and a lot.
The Ink Trader will most likely disappear, so easy ink conversion will be no more, materials will become more expensive, which leads to more expensive glyphs, or just another inflation and a lot of Scribes not bothering with it anymore.
You're never prepared enough, your stock is never big enough and you will profit from cataclysm like no other.