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This blog says that Bruce R McConkie wrote it, but there is no source to back that up
This Oct 1981 Ensign article published about the new edition to the Book of Mormon says:
As always, inspired direction came from the Scriptures Publication Committee of the Quorum of the Twelve: Elder Thomas S. Monson, chairman, Elder Boyd K. Packer, and Elder Bruce R. McConkie, with Brother W. James Mortimer serving as executive secretary.
So it is possible that Bruce R McConkie wrote it but it could've been someone else or a collaborative effort.
I believe the best you'll find is that it was written 'by The Church'