I would recommend the following book:

Improve Your Teaching! An essential handbook for instrumental and singing teachers. By Paul Harris.

This book is extremely helpful fo teachers old and new. It provides an insight into how we teach and how pupils learn. I consider myself to be a thorough and competent teacher but found many new ways and ideas of doing things. The book covers lesson preparation, aural and memory work, effective practice, improvisation and compostition, sight-reading and group teaching. Paul Harris is a well known author, composer and specialist in musical education books and his knowledge and advice are very helpful and insightful.

I would recommend this book to anyone and everyone!