See the "References" section in the GENMOD chapter of the SAS/STAT User's Guide and also these references:

References from SAS

• Allison, P. D. 2012. Logistic Regression Using SAS: Theory and Application, Second Edition. Cary, NC: SAS Institute Inc.
• †Littell, R.C., Stroup, W.W., and Freund, R.J. 2002. SAS for Linear Models, Fourth Edition. Cary, NC: SAS Institute Inc.
• †Morel, J. G. and Neerchal, N. K. 2012. Overdispersion Models in SAS. Cary, NC: SAS Institute Inc.
• Stokes, M. E., C. S. Davis, and G. G. Koch. 2012. Categorical Data Analysis Using the SAS System, Third Edition. Cary, NC: SAS Institute Inc.
• Vonesh, E. F. 2012. Generalized Linear and Nonlinear Models for Correlated Data. Cary, NC: SAS Institute Inc.
• †Zelterman, D. 2002. Advanced Log-Linear Models Using SAS. Cary, NC: SAS Institute Inc.

†These books are no longer in print from SAS Institute but might be available from other sources.

General References Not from SAS

• Diggle P.J., Heagerty P.J., Liang K-Y, and Zeger S.L. 2002. Analysis of Longitudinal Data. 2d ed. New York: Oxford University Press.
• Dobson, A. 2001. An Introduction to Generalized Linear Models. 2d ed. London: Chapman & Hall/CRC.
• Fitzmaurice, G.M., Laird, N.M., and Ware, J.H. 2004. Applied Longitudinal Analysis. New York: John Wiley & Sons.
• Gill, Jeff. 2001. Generalized Linear Models: A Unified Approach, 07-134. Thousand Oaks, CA: Sage Publications.
• Lindsey, J.K. 1997. Applying Generalized Linear Models. New York: Springer-Verlag.
• Lindsey, J.K. 1999. "On the use of corrections for overdispersion," Applied Statistics, 48(4), 553-561.
• Molenberghs, G., and G. Verbeke. 2005. Models for Discrete Longitudinal Data. New York: Springer.