A good Ectomancer has a sensitivity to spiritual energies and apparitions—ghosts. 
In the series
In Grave Peril, a well known ectomancer, Mortimer Lindquist, is leaving town. Lindquist is scared of some super ghost that is coming. He has a theory that the barrier between this world and the Nevernever is slowly being torn away. He says that the border has been waggling back and forth for the last two weeks. The instability has been making it easier to form bigger, stronger ghosts when people die. Soon, the border will be so thin that there will be more problems with demon attacks than Gang violence.
In Dead Beat, Harry Dresden visits Mortimer Lindquist to learn what he knows about the threat of the necromancers in town. While there, he tells Lindquist that he thinks he is one of the most powerful ectomancers that he knows and that Lindquist could do a lot of good in the fight against the Red Court vampires having access to all kinds of informations via ghosts and spirits. 
In Ghost Story, Dresden's ghost visits to Mortimer Lindquist to try to enlist his help to communicate to the people who can help him solve his own murder. While there, a gunman comes to abduct Mortimer for the Grey Ghost. Lindquist calls several ghosts to him including Sir Stuart Winchester, and with their aid, Lindquist defeats the gunman. Later, the Grey Ghost—Corpsetaker—does kidnap Lindquist for the purposes of taking over his body for the use of his magic. Because Corpsetaker needs Lindquist's permission, she tortures him believing he'll relent eventually.