most useful health book!
The most useful reference book I found for on the road and in preparation beforehand (the most important part!) is Traveller's Health by Dr Richard Darwood. No personal links to the author -but the two things that make it unusual is: rather than a single medical author "explaining in layman's terms"- it's a collection of articles from experts covering the particuliar topics that are likely to effect travellers (malaria,prophylaxtic regimes,various diseases and injuries a traveler is likely to encounter etc) so is more thorough and the author runs a well-known travel pharmacy so has regular feedback on what befalls the unlucky. A definite bedside book before any trip.
The Hillwalker's Guide by Constable Guides-second part of which deals with first aid in a more usefull approach than just "maintain ABC's and get to medical help" approach of most health/medical books.
For a really basic-easy to grasp guide to principles of diagnosis etc (although not much on road accidents) is "where there's no doctor"-Hesperian foundation does an online version. defintely three worth reading- a course concentrating on injuries incured in a road collision would be usefull as well I guess. (On a bike trip rather than a large group-it's prudent that everyone makes the time to be familiar with procedures for medical emergencies.)