The Nishikigoi Manual of Diseases & Varieties contains a brief history of koi and details koi health, beautiful quality photographs, and concise information in both Japanese and English. There is a detailed section on remedies, some of which are not contained in other koi health books.
The author , Mr Kuroki was the long time president of the Japanese Koi hobbyist organization, the Zen Nippon Airinkai association. This text should be on every serious Koi hobbyist's bookshelf.