What is the significance of having the made-up Terrance Mann in the film, Field of Dreams, as opposed to J.D. Salinger as the reclusive writer...

I think that the very idea of Salinger appearing in film
would have caused a litigious nightmare that the producers would have not wanted for the
film.  I think that Salinger was so protective of his privacy that to be included in
film and in such a public way would have been a vision of nightmarish proportions for
the producers. They must have understood this.  Salinger threatened legal action through
his lawyers against Kinsella in the publication of Shoeless Joe. He
backed off of this claim because of financial reasons.  However, one also would have had
to figure that the mercurial Salinger saw something in the last name of the author as
being mentioned in one of his own writings decades before.  He relented on this end to
be a character in the book, but instructed his lawyers to specifically prevent his
likeness from being present in a film of the book.  The filmmakers understood this,
reason being why Terrence Mann represents every bit of Salinger in the film.  The book's
depiction of Salinger is a bit gentler, but the desire for reclusivity is all there as
well as the disenchantment that he feels about the results of his writing.  Terrance
Mann is also perfect because he is of a different race than Salinger, helping to enhance
both the differences as well as the similarity to him.  Salinger probably did more to
enhance his publicity by demanding not to be in the film, as nearly everyone watching
understood who was being portrayed.  To this end, it is significant that Mann is in the
movie and not Salinger himself.