Someone on the MedStats discussion group asked about books discussing Poisson regression. If you want to use Poisson regression, you need an overview of the generalized linear model. The classic reference:

• Generalized Linear Models. P. McCullagh, J.A. Nelder (1983) London: Chapman and Hall. [BookFinder4U link]

is quite old, but the book is still worth reading. More recent books include

• Modelling Frequency and Count Data. J.K. Lindsey (1995) Oxford: Claredon Press. [BookFinder4U link] and
• Regression Models for Categorical and Limited Dependent Variables. Scott J. Long (1997) Thousand Oaks, CA: Sage Publications. [BookFinder4U link]