Since this isn't on the Spoiler forum I'm going to flag this all as spoilers:
To get the dope on the Fomor, read the following two short stories:
Even Hand (in Dark and Stormy Knights)
Aftermath (in Side Jobs)
Even Hand is told from the point of view of the man we call John Marcone. No, it doesn't give his real name, it just reveals that Marcone isn't his real name. In it he gets briefed on who and what the Fomor are. We learn that they signed the Accords, have a King, and have various titles... And that Marcone isn't going to give them back a preschooler that they want to experiment with. The child was rescued by Justine and (if I recall correctly) spends most of that story wearing only a towel and Marcone is willing to pay a weregild if he has to in order to avoid sending the kid back. Yes, he knows that a certain WCV is trying to manipulate him (even if Justine hasn't clued into the fact) and does a cost benefit analysis of killing Justine to send a message while saving the kid.
Aftermath is told from the point of view of Murphy. It's set 30 minutes after the end of Changes and the main theme of the story is "Harry is gone and Murphy has to deal with the aftermath of his disappearance" - mainly because Harry told a lot of people if they couldn't reach him they should go to Murphy so she could help them. It shows that the Fomor were moving into position as the shitstorm that claimed the RC was forming. Sensing the massive magical build up the Fomor knew that either the RC or the WC would be gone and was ready to take action. As in hiring bounty hunters to steal some werewolves and a host of minor talents before they knew that Harry was gone.
Those two stories add quite a lot to the Fomor as shown in Cold Days. To echo others, for more info on the Fomor, head to the spoiler forum for the books - they've had countless threads on the Fomor and will probably have countless more. Just stay away from Ms. Duck - her theories get weird (and are sometimes close to what Jim is doing).
Richard