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.
Sadly, I’m going to miss the Pacific Northwest Congress this year. It was excellent in 2005 featuring Canadian Olympic Coach Tony Smith and many, many great presenters. 
Gymnastics Coaching.com 