Book of Mormon/As a "familiar spirit"

10. If the words "familiar spirit" in Isaiah 29:4 refer to the Book of Mormon, why does "familiar spirit" always refer to occult practices such as channeling and necromancy everywhere else in the Old Testament?

The comparison does not say that the Book of Mormon is a familiar spirit, but that the message from the Book of Mormon would be comparable, or like such a spirit.

The question ignores the Bible writers' beliefs about "familiar spirits." Such spirits represented the dead, who had passed on and yet could give a message of importance to the living. The NET Bible translation renders this verse as

"Your voice will sound like a spirit speaking from the underworld."

Thus, the Book of Mormon, being a record from a fallen Christian civilization, would be "as if" the dead spoke, since those who are now dead can speak to us. (All writing from another time does this—it allows the dead to speak to us. Matthew and Paul speak to us "as if" from the dead in the Bible; Shakespeare speaks to us through his plays.)

This doesn't mean that Isaiah was only referring to the Book of Mormon, or that he was particularly thinking about it at all. Nephi simply used the imagery and language of Isaiah, and adapted it to make his point. This was common practice in the ancient world. One wonders how young Joseph Smith knew that.