Neumann's book on ornamentation in baroque music was reviewed by Neal
Zaslaw in Early Music (1981):63-68; the Mozart volume was reviewed
by Stanley Sadie in Musical Times (1988):128-9 and by Robert Levin
in Journal of the American Musicological Society (1988):355-68.  None
of the reviews is entirely positive.  The main problem with
Neumann's work is his unwillingness to imagine that the music might
go differently than the way he knew it went before he looked at all
that evidence.  If Mozart wrote out some embellishment, than the
passages that he left plain shouldn't be embellished -- what you
see is what you get.  Nonsense.
Zaslaw pointed out that the earlier volume, while it is extensive,
ignores some pretty important topics:  passaggi (as opposed to
small ornaments indicated by signs), English ornamentation, composers
and practices far removed from Bach, and rhythmic alteration (about
which Neumann engaged in debate with David Fuller and others for
more than a decade).
But the diversity that Neumann does document shouldn't inhibit or
paralyze us.  It demonstrates that there are usually no absolute
or definitive answers to particular problems of performance practice.
As Zaslaw says, "This is not to say... that 'anything goes'.  But
given the knowledge, taste and ability to keep within the boundaries
of a specific style, quite a lot goes."  Just because that sort
of answer is not typically found in the teaching studio does not
mean that we have to shy away from it!  Students can still get
it quite wrong, and stand in need of correction, even if we
cannot (and should not) offer a definitive and singular way of
reading a score based absolutely on chapter and verse of one
treatise or another.
Do your homework.  Relax and play the music.
Sandra Mangsen
University of Western Ontario