Just what are familiar spirits?
According to Keil and Delitzsch Commentary conjuring familiar spirits was dabbling in necromancy...communication with the dead.
Lev. 19:31. True fear of God, however, awakens confidence in the Lord and His guidance, and excludes all superstitious and idolatrous ways and methods of discovering the future. This thought prepares the way for the warning against turning to familiar spirits, or seeking after wizards. אֹוב denotes a departed spirit, who was called up to make disclosures with regard to the future, hence a familiar spirit, spiritum malum qui certis artibus eliciebatur ut evocaret mortuorum manes, qui praedicarent quae ab eis petebantur (Cler.). This is the meaning in Isa. 29:4, as well as here and in Lev. 20:6, as is evident from Lev. 20:27, "a man or woman in whom is an ob," and from 1 Sam. 28:7, 8, baalath ob, "a woman with such a spirit." The name was then applied to the necromantist himself, by whom the departed were called up (1 Sam. 28:3; 2 Kings 23:24). The word is connected with ob, a skin. יִדְּעֹנִי, the knowing, so to speak, "clever man" (Symm. γνώστης, Aq. γνωριστής), is only found in connection with ob, and denotes unquestionably a person acquainted with necromancy, or a conjurer who devoted himself to the invocation of spirits. (For further remarks, see as 1 Sam. 28:7ff.).