One our members, Ron C, recommended this book and I have found it to be my go-to book when I have questions about my Koi. Here is a link to it on Amazon:

Koi Health & Disease: Everything you need to know about Koi 2nd Ed. 

Doctor Erik L. Johnson, D.V.M explains in easy to understand terms the various health challenges a Koi Keeper will face and provides helpful insight into how to remedy and treat the problems. He also provides instructions on how to deal with pond issues that arise. It's a great reference guide and I would recommend it to anyone who has Koi - both beginners and experts. 

What reference books would you recommend?