Sequel to City of Masks, the setting is again Talia, the parallel world very similar to 16th-century Italy, but the main character in this book is Georgia - who has a love of horses. She is desperate to buy a little, dusty winged horse that has appeared in a local antique shop. This tiny, winged horse proves to be the talisman that transports Georgia right into the rivalries and the high-octane excitement of the hugely competitive Stellata horse race. Mary Hoffman proved herself a mistress of a narrative tour-de-force with City of Masks and this sequel will not disappoint. Fans will be thrilled too that favourite characters from the first book, such as Lucien and Arianna, again appear.
City of Stars - Mary Hoffman

i found it on amazon it is the plot