[0] http://www1.idc.ac.il/tecs/ [1] http://csapp.cs.cmu.edu/
CSAPP is unfortunately a hulking $100 hardback, but the authors have put close to 200 sample pages online. Despite looking a bit fearsome, it's extremely clear. And TECS is just awesome--they take you all the way through building (in a hardware simulator) a simple 16bit computer!