Discussion on the theory of the Sagnac
interferometer
and the Michelson-Gale experiment can be found here.
An interactive Sagnac interferometer simulator can
be
found here.
The original paper by Georges Sagnac can be found here.
The Dufour and Prunier paper (English Translation)
with
its detailed analysis of the Sagnac interferometer and how it does not
obey relativistic theory can be found here.