Koi Health and Disease - Erik L. Johnson, D.V.M. 

If there is a book that a Koi keeper must have it's this book.  EXCELLENT FOR BEGINNERS...Without sacrificing plenty of depth where needed... a Koi keeper's bible!
This book goes into extreme detail and gives you STEP by STEP instructions on how to save fish lives under a hundred different conditions.

You'll find it very impressive with its content and the excellent balance between practical and technical information. 

There is a great deal of information on parasites, bacterial pathogens, fungal infections, and viral infections. Primary and alternative treatment protocols are very well presented. Microscopy is well detailed.

Also See our Koi Diseases Section for additional reading.