G Agha, I. A. Mason, S. Smith, and C. L. Talcott. A Foundation for Actor Computation. Journal of Functional Programming, ?(?), 1993. available as postscript from http://maths.une.edu.au/~iam/Data/Papers/96jfp.ps.
William Douglas Clinger. Foundations of Actor Semantics. MIT Artificial Intelligence Laboratory TR 633. May 1981.
Some insights at this wiki.
Mark Miller has much more info.
A Wikipedia article