Müller/Rusch - String Method, Book 1 - Viola - Kjos Music Co

I know this book is old-fashioned, but I use Müller-Rusch books 1-3 (as appropriate) with young students. There is a thorough introduction to the instrument, including a well-paced introduction to new notes. Book 3 has the gentlest introduction too 3rd position that I've seen anywhere. Also it has helpful instruction for developing a wrist vibrato. I don't move on to the later books in the series; so I can't comment on those.