"Co-authored" would refer to the original collection, English Folk Songs from the Southern Appalachians: comprising 122 songs and ballads, and 323 tunes collected by Olive Dame Campbell and Cecil J Sharp (Putnam, New York, 1917). This was published only a year after Sharp's collecting expedition. Olive Dame Campbell had collected the songs which she contributed rather earlier (1907-1910) and she didn't accompany Sharp and Maud Karpeles on their field trip, though her husband John went with them on the first journey, and both the Campbells provided invaluable help with planning, advice, contacts and so on. It is unlikely that Sharp would have visited the region had Mrs Campbell not brought her material to him and offered him help and encouragement; however it is the case that he and his assistant Maud Karpeles did the vast bulk of the collecting.

The next edition (virtually a new book, though issued under much the same title and containing almost all of the material in the first collection together with a great deal more, some of it collected in subsequent, longer trips in 1917 and 1918) appeared as (full title) English Folk Songs from the Southern Appalachians collected by Cecil J Sharp: comprising two hundred and seventy-three Songs and Ballads with nine hundred and sixty-eight Tunes, including thirty-nine Tunes contributed by Olive Dame Campbell, edited by Maud Karpeles (Oxford University Press, 1932). Later editions specified 274 songs and ballads; maybe there was an error in the count in 1932.

Sharp had died in 1924 and had no hand in the new editions or the re-jigging of the title. His original introduction was retained, but was shortened a little; whether references to Mrs Campbell were reduced I don't know, having seen neither the 1917 nor 1932 editions (some further revisions were introduced in the next edition of 1952), but if her significance as an inspiration and pioneer collector in the region was underplayed in 1932 and later, that will have been down to Maud Karpeles, not to Cecil Sharp.

Her contribution in terms of material to even the first collection was relatively small, and her importance at that time seems to have been more as a groundbreaker, enabler, advisor and generous friend (she wanted the collecting done, and she felt that Sharp was the man to do it). As the pioneer collector in the region, her name got equal billing on the original edition (if we assume alphabetical order; pride of place otherwise. At all events, this was at Sharp's initiative), but maybe Miss Karpeles felt that the revised title reflected more accurately the relative contributions made: that I don't know, of course.

Olive Dame Campbell has disappeared rather in Sharp's shadow; not particularly because he was a man, but because he was a giant in his field. A full re-evaluation would be welcome and informative, and I hope that this is a step towards it.

See also Mike Yates' paper at Musical Traditions:

Cecil Sharp in America: collecting in the Appalachians