Epidemiology and Prevention of Vaccine-Preventable Diseases, 11th ed.
Chapter 6 covers influenza. Scroll to "Download" to read for free.
Centers for Disease Control and Prevention. Epidemiology and Prevention of Vaccine-Preventable Diseases. Atkinson W, Wolfe S, Hamborsky J, McIntyre L, eds. 11th ed. Washington DC: Public Health Foundation, 2009