Cotton / edited by David D. Fang and Richard G. Percy. (OCLC #909779516)

When catalog electronic resources, I follow the Provider-Neutral guidelines (even if I only know of one instance of that resource) and label the provider in the 856 ǂ3:

856 40 ǂ3 ACSESS Digital Library
  ǂu http://dl.sciencesocieties.org/publications/books/tocs
      /agronomymonogra/agronmonogr57