First published in 1958 under the pseudonym David Osborne, Invisible Barriers is an expansion of the short story "And the Walls Came Tumbling Down" (1957), which was published under the author's own name.
Used availability for Robert Silverberg's Invisible Barriers
August 2013 : UK Kindle edition
Title: Invisible Barriers
Author(s): Robert Silverberg
Availability: Amazon UK