My top ten would include:
Worms,
Coccidiosis,
Mycoplasma Infections (MG - includes CDR),
Infectious Bronchitis,
Laryngotracheitis (LT),
Newcastle Disease,
Avian Adenovirus Infections,
Infectious Coryza,
Avian Influenza &
Marek\'s Disease.
The Salsbury Manual of Poultry Diseases is out of print, but a great resource if you can find one.
Peter Brown, of First State Veterinary Supply Inc., advertises in the Poultry Press. I have talked to him several times and would suggest you get his input.