1. J. R. Schrieffer, Theory of Superconductivity, W. A. Benjamin, Inc., New York, 1964.
2. P. G. de Gennes, Superconductivity of Metals and Alloys, W. A. Benjamin, Inc., New York, 1966.
3. K. Maki, Gapless Superconductivity, in: Treatise on Superconductivity, ( by R. D. Parks, ed.), Marcel Dekker, New York, 1968. This reference contains a rather complete list of contributions to the subject.