G Agha, I. A. Mason, S. Smith, and C. L. Talcott. Towards a theory of Actor Computation. In Concur '92, volume 630 of Lecture Notes in Computer Science, pages 565-579. Springer-Verlag, 1992. available as postscript from  http://maths.une.edu.au/~iam/Data/Papers/92concur.ps.

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