Diana Wynne Jones' Fire and Hemlock tells the story of a girl, Polly, and the unlikely friendship between her and Thomas Lynn, a cello player for the London Philharmonic. At the opening of the book, Polly realizes she has forgotten her entire friendship with Tom and she goes back in her mind to when she met him at age nine. This book is full of allusions to different works and is based off a ballad. Telling you the name of the ballad would spoil the plot though. If you liked this, I'd recommend Diana Wynne Jones' other books particularly The Merlin Conspiracy or the Chrestomanci books.