There is a collection of Auden's libretti available; i don't think it
ever came out in paperback so you'll probably have to check a fairly
large library.
It does seem odd that his libretto might have been rejected as
"commercially nonviable" since he was an experienced theatrical writer
and not entirely a snob about it.
Lang Thompson
Screen-L is sponsored by the Telecommunication & Film Dept., the 
University of Alabama.