Okay I went through the first 7 books looking for any information that might lead us to the killer ... obviously nothing definitive here, but thought it was worthwhile on a dull Saturday morning...

EoTW clues:

Slayer
Isam and Luc’s childhood stories told briefly

Graendal
Hints that the Forsaken are loose but no mention

TGH

Slayer
Shadow Prophesy alluding to Isam and Luc merging into 1 being

Graendal
More hints about the Forsaken being free but nothing specific

TDR

Slayer
No mention

Graendal
No mention

TSR

Slayer
First mention of the name Slayer
We learn from the Wolves that he can enter TAR in the Flesh (by unknown means), and that he kills wolves forever when there
We learn that Lord Luc and Isam are both in the TR and learn that they are one and the same, or at least can infer from the various big hints
We know that the Myrddraal fear being found by Isam
We learn/see that at least Lord Luc is great fighter, he's been killing Myrddraal all over the TR
We learn that Fain thinks Isam is a "problem for another time"

Graendal
First onscreen mention
We learn she takes “trophies”

Misc
We learn Balefire can destroy the Forsaken
We learn that the Forsaken are not all a united front, and that even amongst the BA there are different orders being given

TFoH

Slayer
No Mention, probably off convalescing from his wounds given by Perrin in TSR

Graendal
First on screen appearance
We learn about her Slaves (Power and Beauty and the heavy use of Compulsion)
We learn that she finds it odd that the Forsaken aren’t actively trying to kill each other
We learn she thinks she is above mere humans
Graendal is aggressive enough to challenge Lanfear in front of other Forsaken and would likely use Balefire in such a fight
We learned that she had carved out a power base and would protect it rather than sitting around plotting against Rand
We learn that Asmodean knew she was in Arad Domon
We learn she pokes around in TAR, spying on the other Forsaken
We know she is aware that Asmodean is teaching Rand and that she is part of the plot to kill/control Rand that Lanfear, Sammael and Rhavin are all part of as well

LoC

Slayer
No mention

Graendal
We learn she remains in cahoots with Sammael and is giving him information on the other FS
We learn that she tends to hide things in plain sight
We learn she already knows of Lanfear and Rahvin’s deaths, and assumes Moghedien and Asmodean are also dead at Rands hands
We learn that Sammael did NOT know that Asmodean was dead
We learn that Graendal is willing to go to great lengths (go to Sharra and steal the rulers) to achieve even small goals
We learn that she hates being threatened in anyway
We learn she is frightened of Rand and will go to great lengths to avoid his detection
We learn that she was a master of mindgames and also some sort of Psychiatrist in the AoL

Misc

The DO already also knew of Asmo’s death


PoD
We learn Graendal looted Sammaels rooms in Ilian