This is a great series of books. My son who is 9 loves the mystery element and read all 17 books in a few weeks. My daughter is 13 and while she is probably a bit old for the story lines claims them as 'background reading' whilst studying Latin. I at 40 find the plots a bit predictable after a while but enjoyed reading the books regardless. There is much to be gleaned of the Roman way of life and some useful vocabulary picked up. The BBC have serialised these and the dvd is now on the christmas list!
In this book the children help one of Mirian's childhood friends.