1. For an historical review including brief technical descriptions of such theories in the early part of the 20th century see: hubert F. M. Goenner, 7, (2004), Online Article: cited Jan 10, 2012, http://www.livingreviews.org/lrr-2004-2.
2. A Classical Field Theory of Gravity and Electromagnetism
3. An Introduction to Differential Geometry;Eisenhart,1947
4. Gravitation and Cosmology;Weinberg,1972