I used to play AP Nasus a long time ago before his Spirit Fire AP ratio was nerfed.
Lich Bane works very well with Malzahar since he wants AP anyway and has a good attack animation. Malady contributes to damage from his spells with both the AP and the MR shred. Frozen Mallet along with the bonus speed from Lich Bane allows him to chase more effectively, so that he doesn't have to rely on Malefic Visions to kill fleeing enemies. Plus, since Malzahar often brings Ignite which has a relatively low cast range, the bonus health from Frozen Mallet allows him to survive mid-range encounters more easily, especially since he is vulnerable while channeling his ultimate.