In my quest to list the (surprisingly few) excellent books for coaches, I give the thumbs up to Rhythmic Gymnastics by Nadejda Jastrjembskaia and Juri Titov.
As with many Russian authors, I find this book overly academic and methodical. No need to read every word. But if looking for a few good ideas to improve your acrobatics program — it makes an excellent browse.
I went first to the Developing Flexibility chapter as this is where I have most to learn from Rhythmic Gymnastics. The text advocates using a combination of static (1/3) and dynamic (2/3) stretching drills “avoiding abrupt or jerky movements”.
Other key points:
girls flexibility increases drastically between ages 7 and 10 — and is maximized between 11 and 13 do flexibility improvement exercises at the end of a vigorous warm-up and at the end of the work-out
There is not much mention of the excellent musical warm-ups typical of Rhythmic training. (The Canadian Men’s National team has a similar dynamic warm-up though not to music which is excellent.)
This one is widely distributed so you may find it in your local library.
Gymnastics Coaching.com 