Yemen Divided: The Story of a Failed State in South Arabia by Noel Brehony.

Noel served as a British diplomat for some time as ambassador to the People's Democratic Republic of Yemen before unification in 1990.

It's a decent book, and, as far as I am aware, the only book that deals exclusivley with the state of South Yemen. An Amazon reviewers states that it does read as a laundry-list of names, dates, groups etc. and I would agree with this; it is not exactly so easy to read at times.

But I think it would still serve well for your purposes, and has an extensive bibliography that may point you to other sources which may be of interest to you.