A book that I've found invaluable is The Poultry Farmer's and Manager's Veterinary Handbook, see Amazon http://www.amazon.co.uk/gp/product/1861262612/ref=as_li_ss_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=1861262612&linkCode=as2&tag=poultrykeeperforum-21
As the title suggests, this is written mainly for commercial poultry farmers, but nevertheless it succeeds in being very readable and clear and useful to those of us who just keep a few birds and want to know how to look after them properly. It emphasises good management practices and disease prevention strategies, and also goes into detail of how to diagnose diseases and infestations and how they can be spread and treated. I've found it a really interesting handbook, useful to anyone, whatever their level of expertise or interest.
Two more that I enjoyed are by David Scrivener, who is a Poultry Show judge and very experienced breeder, Popular Poultry Breeds and Rare Poultry Breeds. See http://www.amazon.co.uk/gp/product/1847971032/ref=as_li_ss_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=1847971032&linkCode=as2&tag=poultrykeeperforum-21
And http://www.amazon.co.uk/gp/product/1861268890/ref=as_li_ss_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=1861268890&linkCode=as2&tag=poultrykeeperforum-21
Both books take you through the history and description of each breed, with excellent colour photos. His style is interesting and lively, and these books are worth a read whether you are just wanting to look up something about a specific breed, or are thinking of getting a breed you haven't experienced before.