I found a fantastic book that every voice teacher should have in their collection, it is called The Private Voice Studio Handbook, written by Joan Frey Boytim. The author is well-known for her many solo compilation books and it is very likely that any one who has studied voice in high school or college (or privately) has used her books.
Just a short review of the book (I’ll do a more detailed one at a later date): It is written as though a woman was writing to Mrs. Boytim seeking advice on starting a voice studio. The book consists of her questions and the replies written by Boytim. It includes such topics as repertoire selection, managing a studio, what to charge, how to do taxes, and more. At the end of the book she also includes pages of warm-ups and sight-reading exercises for use by the teacher in their own voice studio (she gives permission to copy them for students). I would like to share a sample of two of the very useful pages that have helped me tremendously, the warm-ups and some syllable sheets (there are 5 of those)…after all, warm-ups are sometimes difficult to think up on your own! She also recommends using the Concone sight-reading exercises and Marchesi (she specifies what age etc. to use them for, and both can be found free at imspl.org). The Syllable Sheet also comes with accompaniment parts for the teacher to play along with the student. The accompaniment has some dissonant chords, to aid the student in learning how to maintain their own note with other music going on.
I highly recommend purchasing this book! It is a must-have for anyone considering teaching private voice lessons. Enjoy!