Books on Poultry diseases

Can anybody recommend a good book (or two) on poultry diseases?  Would prefer if it covered chickens and ducks but not essential. 

Thanks in advancev

Lots around, some expensive, but few useful. The main factor in spotting disease signs is knowing what is normal. Always a problem and only solved by much observation of healthy animals. This is why well trained vets who do not specialise in birds are not keen to get involved. The word 'Disease' today is used a synonym for infection whereas originally it came from dis - ease and to most chook keepers (like me) it is hard to know when the animal has an infection, is going through a life episode (moulting/broody etc), has had an accident or is suffering from environmental problems such as bullying or pests.
The Haynes manual is useful as is Katie Thears, but once having looked at those, you'll probably want to search the web for other ideas and opinions on disease. Both books are useful for advice in other areas and as you can use them in wet and dirty places both are worth having to hand.
There are lots of sites e.g http://www.chickenvet.co.uk/general-health-and-common-diseases/parasites/index.aspx
which will help sort out problems, but I'd suggest that having a nice warm quiet place and a quarantine place is probably the action to take now whilst all is well as you don't want to be trying to sort these out when you're worried about a bird.
Here's wishing that any book you buy lies dust covered on the shelf having never been needed.