These verses deal with the seeking of advice from soothsayers, wizards, and those with familiar spirits. These are the astrologers and psychics of Isaiah’s day. The people believed in them more than in the Lord. This is another example of their rejection of a higher power. As Isaiah asked, ’should not a people seek unto their God for the living to hear from the dead?’ The Lord is the only source of true light. If the people seek wisdom from those with no light, they will only behold ’darkness and dimness of anguish.’
From the 1981 Old Testament Institute Manual, pp 145-6:
"The expression ‘familiar spirits’ is not an accurate term to convey the significance of the Hebrew term used anciently. The Hebrew word ob means ‘a leather bottle or bag’ (see William Gesenius, A Hebrew and English Lexicon of the Old Testament, p. 15). This object was used by the practitioners of necromancy, a deceptive craft of pretended communication with the dead. The art involved a kind of ventriloquism wherein the voice or message of the ‘departed spirits’ was called forth from the bag or sometimes a pit. (See G. Johannes Botterweck and Helmer Ringgren, Theological Dictionary of the Old Testament, 1:131, 133-34) The peeping (chirping) and muttering (twittering) somewhat like birds was intended to invoke the departed spirits or to convey the pretended message (see Young, Book of Isaiah, 1:318). The Lord warned Israel and Judah of such deceptions early in their history (se Leviticus 19:31; 20:27; Deut 18:10-11). President Joseph Fielding Smith in commenting on these ancient practices gave this warning that applies even today:
"To seek for information through…any way contrary to the instruction the Lord has given is a sin. The Lord gave positive instruction to Israel when they were in the land of their inheritance that they were to go to him for revelation and to avoid the devices prevalent among the heathen nations who occupied their lands…
“All through the Bible, the New Testament as well as the Old, the Lord and his prophets have expressed their displeasure when the people turned from the Lord to ’familiar spirits.’” (Answers to Gospel Questions, 4:33)
“To those in quest of spiritual light, this word of counsel: Seek it only in the Lord’s appointed way. Follow the advice of the Apostle James and the example of Joseph the Prophet. Never go upon the Devil’s ground. Keep away from all deceptive influence. One may believe in hypnotism, without being a hypnotist, without surrendering one’s will to the will of the person exercising that power—a very dangerous power when wielded by an unprincipled possessor. In like manner, one may believe spiritualism real, without becoming a spiritualist, without attending ’séances,‘ without consulting ’mediums,’ without putting trust in planchettes, ouija boards, automatic pencils, false impersonations, or in any way encouraging the advances of designing spirits, who thus gain an ascendancy over their victims, leading them into mazes of delusion, and often into depths of despair. Go not after them; and if they come to you, put them to the test. ’Try the spirits.’ (1 Jn. 4:1) If they speak not according to revealed truth, if they conform not to divine standards, ’it is because there is no light in them.’ (Isa. 8:20)” (Saturday Night Thoughts [Salt Lake City: Deseret News, 1921], 311 - 312 as taken from Commentaries on Isaiah in the Book of Mormon, ed. by K. Douglas Bassett, [American Fork, UT: Covenant Publishing Co., 2003], 146)