In the impossibly thrilling The Scorpio Races, Stiefvater — the 29-year-old author of the Shiver trilogy — turns to the Celtic myth of the fearsome water horse, the capall uisces, for inspiration. Each November the island of Thisby hosts the ''Scorpio Races,'' which use not horses but capall uisces, more interested in murdering their riders than in crossing the finish line. Our two heroes are the tomboyish Puck Connelly, the first woman to enter the race, and the quiet loner Sean Kendrick, capall whisperer. Though a potent romance brews between them, the question is less ''Will they or won't they?'' than ''Will they both survive?'' With this beautifully executed drama, Stiefvater has established herself as one of the finest YA novelists writing today. A-