One excellent medical reference is

Ferrets, Rabbits and Rodents - Clinical Medicine and Surgery, by 
   Elizabeth Hillyer and Katherine Quesenberry (1997)

Another good reference work, a bit outdated but still worthwhile for
both vets and others, is 

Biology and Diseases of the Ferret, by James G. Fox.  Lea and Febiger,
   Philadelphia (1988).  ISBN 0-8121-1139-7.

There is also a series out by the

          American Animal Hospital Association
          12575 West Bayaud Ave.    
          Lakewood, CO  80228
          tel. 800-252-2242  

for practitioners on exotic pets.  There are five books in the series.
Dr. Jeff Jenkins and Dr. Susan Brown produced the one on Rabbits and
Ferrets (he did the rabbit part).  Many people feel that it is
practical and useful; it has drug dosages, treatments, husbandry,
normal clinical pathology values, and diagnostic techniques that might
be useful for your vet.