Behind the Glass... Howard Massey
The Art of Mixing (I forget the author...very cheesy..but good stuff)
As far as specifically pertaining to mixing and mastering hiphop...there's not much out there...but you should be able to translate these ideas into any genre. Pop over to my forum if you have specific questions...
Elements of Mixing - Social network for aspiring mixers
It's no GearSlutz...but it is specific to Rap and HipHop.
Peace!