How to Get It
You are looking for books marked "The Schools of Arcane Magic." They all have that in the name and then a volume name:
When you click on the book, it will remain for 3 minutes (so you don't have to fight with someone else over it unless there's a long line). Then it will disappear and stay gone until it respawns roughly 3-4 hours later.
WHERE Are These Books?
They share spawn points with useless books but each book is restricted to only one location (so once you have it, you can move to a different spot). The useless books appear more often than the one you need.
Click on the images to see a full size screenshot. I tried to include reference details so you don't get turned around.
Introduction: Across the street from Krasus Landing. Entering from the street, not the square, you'll see Archmage Celindra (she's facing the entrance and thus you) and the shiny blue triangle portal. Look at the bookcase on your right and the book will be on the floor in front of it.
Conjuration: Violet Citadel. Enter, turn north, the one on the left.
Enchantment: Threads of Fate (across from Violet Citadel). Enter, go upstairs, exit to balcony, turn right. On a crate.
Necromancy: Go upstairs in the Legerdermain Lounge, enter the room to your left and look on the bookshelf.
Abjuration: Across from (and a little south of) Violet Hold is Dalaran Visitor Center. Enter, turn right, look at the floor by the table between the two bookshelves.
Divination: Violet Citadel. Go upstairs, turn left. It will be on the floor between the bookcases.
Illusion: Enter under the arch of Violet Hold and turn left. Find Archmage Timear and look at the crates beyond him. It's on the left one.
Transmutation: Enter the Legerdermain Lounge, look at the bookshelf near the western door.
And here's a map with all of the book spawn points.
I Got Them All, What Now?
Once you have the achievement, you get a book that portals you up to the guy who gives you the pet. You just have to talk to him and it appears in your bags (so make sure you have a slot).
You cannot get the pet without the achievement, even if a warlock ports you up to the guy. A friend had fun climbing walls to him (or something), but he did it for a screenshot, not for the pet.