Walter Wagentonks book "Principles of Helicopter Flight" has a section on Sling loads and rigging loads to reduce swinging etc. This section of the book was complied by a couple of NZ helicopter Pilots very experienced in this type of work.
Bill Reid another very experienced pilot on sling work writes articles for Heliops magazine, and I think if you go to Nelson Helicopters website you can find these articles in the News section.