Rabbotai,
On daf 18/b we find stories of people communicating with the dead. Shmuel seeks out his father who was niftar, talks to him and gets information about where yesomim's money is and helps Levi get to the Mesivta D'Rekia. It was important information regarding tzedaka but why was there no issur of "doresh el hamisim," involved?
B'kavod and good moed,
Sam Kosofsky
(a) The SEFER YERE'IM (#91, cited in Sefer Beis Yosef) answers that the prohibition of "Doresh El ha'Meisim" applies only when a person tries to communicate with the body of the dead person. It is permitted, though, to communicate with the soul of the dead person, because -- since a soul does not die -- this is not called being "Doresh El ha'*Meisim*."
(b) The MAHARSHA points out that Shmuel's communication with the dead occurred while he was asleep, in a dream (this must be so, points out the Mitzpeh Eisan, because Shmuel was a Kohen and prohibited from entering a cemetary). It could be that communicating with the dead in a dream is not an action of "Doresh El ha'Meisim."
Kol Tuv,
-Yisroel Shaw