The reference in _The Singular Mark Twain_, p. 4 states:

"_The Adventures of Huckleberry Finn_ still raises hackles, especially
of those who cannot read its irony about "nigger Jim."

The index references "nigger Jim" only under _Adventures of
Huckleberry Finn_.

To present the quote only in this context is misleading a reader to
believe that is how Twain referred to Jim in that particular book.  An
in depth discussion of _Huck Finn and Tom Sawyer Among the Indians_
would have added clarity to the present argument.  But that manuscript
is not discussed or listed in the index of _The Singular Mark Twain_.

In both the Fishkin and Mac Donnell reviews, both reviewers
are following the lead presented -- "nigger Jim" as it related to
_Adventures of Huckleberry Finn_.