The Green Bronze Mirror by Lynne Ellison
Karen is playing on the beach when she finds an ancient mirror buried in the sand. She looks into it, and is transported back in time to the Roman empire. Finding herself a slave, she faces many hair-raising adventures in her struggle to return to her own time.
This Long-lost work of young adult Roman fantasy/ historical fiction in the great tradition of Rosemary Sutcliff, written when the author was 14 years old, first published in 1966 and since out of print, has been reprinted in a new paperback edition with BRAND-NEW illustrations by Philip Smiley. The book is NOW available from the publisher's website http://www.cnposnerbooks.co.uk: also from Ebay and http://www.amazon.co.uk