(String Solo). Trott's Melodious Double-Stops Book 1 for Violin (50327290) has been a landmark string exercise book for decades, selling thousands of copies a year. Now the same exercises have been insightfully transcribed for viola.
This starts out easy (the first eight etudes have an open string in every double stop) and quickly progresses to more difficult double stops. It's a great way to get students (or yourself!) more comfortable with double stops.
Minus one star because there are some musically unsatisfying moments where the etude just doesn't sound quite right even when you're playing the right notes.
I'm using this around Suzuki Book 5, post-Wohlfahrt études.