I also really liked The Unremembered Empire. It's Dan Abnett but it's not written the way he usually writes.
The Alpha legion are badass, (I loved that bith with Guilliman seeing a three headed hydra in a cracked window)
Lion el johnson is his usualy dickish self (that bit with the space wolves ritual dual made me laugh) and Nighthaunter is uber badass as some kind of daemonic batman.
The xanthites (and the Kabal for that matter) are doomed to failure. They will turn chaos against chaos, but they fail to notice chaos already fights chaos. (slaanesh vs khorne etc...) In fact chaos is so used to fighting itself that it can't be properly turned against itself or win so hard it ends up destroying itself. (I think that's the main reason why most of tzeentch plans disrupt each other) You can't turn what already has been turned.
This is possibly the reason GW retconned Mallal out of the chaos pantheon. There is no need for a chaos god who fights the other four, when those four are already fighting amongst themselves...