I did some work with them a few years back with less than stellar results. There is a thread here with a little information and a book recommendation. http://bonsainut.com/forums/showthread.php?t=23
The book is older, my first edition is from 1965, and is rather difficult to follow at times. I suspect this is due to the translation from Japanese. It is only partly, roughly half, concerned with bonsai but it is still worthy of consideration if you are seriously considering having a go with Mums.