You would want to have read the earlier Magic Circle/Circle Opens books in this series before this one. This book gets a plus for moving away from the serial killer plot that Tamora Pierce uses in too many of her books but a minus (for me) for having sexual relations (gay and straight) in a youth book. I still very much like the idea of different types of magics and the way the four friends need to work together to achieve their full power. By the way, the "Ages 4-8" note is way off base. It should be "Ages 14-18" or something like that.
It was sad to see how separated the four mages had become over the years, but as each one started opening connections between one another, I was cheered. The story itself was a bit predictable but worthwhile reading to see all four of them back together again.