The SIGGS Theory Model was developed by integrating Set theory, Information theory, Graph theory, and General Systems theory. SIGGS is completely defined in the original publication: Maccia, E.S. and Maccia, G.S. (1966). Development of educational theory derived from three theory models. Washington, DC: U.S. Office of Education, Project No. 5-0638. Please note that any quotations on the SIGGS Web (WWW documents) with page numbers only are from this 1966 printed publication.
Ted Frick has taken the liberty to slightly modify the wording of many definitions in order to improve their readability and to focus the SIGGS Model on 'education systems.' Actually, SIGGS is a general model that could be used to develop theory for many diverse kinds of systems.
Since their theory model is a definitional system, hypertext is an ideal way for people to learn about it.
It also may be useful to begin by reading Restructuring Education Through Technology. For further examples of systems thinking, see the article, Transforming Education. It is probably best to next look at the educational theory, since all properties can be easily linked to from the theory. A recent application of this theory for helping educators to think systemically is a grant proposal for an educational systems simulation, SimEd.