Another great reference that we take on backcountry trips with the Boy Scouts is "Wilderness First Aid", published by the National Outdoor Leadership School (NOLS). It's the text they use with their WFA course. It's also available on their web site:
http://www.nols.edu/store/books/nc_wildfirstaid.shtmlThe book has excellent information on diagnosis and treatment, but, also gets into methods of prevention for situations that you'll encounter. It also deals well with the concept that First Aid in an urban or suburban environment is a lot different, potentially, than in a wilderness environment. In most places where people live, first aid/first responder just has to keep someone stable/alive until the EMS system kicks in. Wilderness First Aid deals with longer term scenarios, and with the need to frequently improvise methods to transport/evac.
Nice treatment of an important topic.