The Four Tomes

In the Shadow of the Light is in the same room as the first boss, near the bottom of the stairs on the left side (coming from the entrance).

Kel'Thuzad's Deep Knowledge is in the Halls of Illusions on a shelf on the left side (this one is hard to miss).

Forbidden Rites and other Rituals Necromatic is in the southwest corner of the Butcher's Sanctum, on top of a table.

The Dark Grimore is in the viewing room, on the highest platform, in the corner on top of a stack of books.

And now you've found all of the books! =D

Edit: These are the locations for the non-heroic mode. I haven't run it on heroic yet, but once I do I will update.

I'm guessing these are randomized, as my Forbidden Rites was in the northwest corner of the room and my Dark Grimoire was on the bottom level on a table.

Just got the quest as part of a random heroic, and then the group was in a hurry so I only got 3 books. The rest of the party split and the remaining mobs were too tough for me to solo. Went back after dying, and the dungeon and epic was now regular level 43 - and couldn't finish the quest. Found the missing books and destroyed them, but the quest didn't seem to register them. So now, I have to basically put together a group at heroic level. Or just abandon, which I will probably do.

At least I got the heroic level 90 armor plate though.

Edit: thanks to post below, found out I could change the level of dungeon before I entered. Thanks to androjeans for actually pointing out information rather than merely downvoting.