Mary Pope Osborne 
Magic Tree House 2: Castle of Mystery 

Eight-year-old Jack and his little sister, Annie, are playing in the woods during their summer holiday, when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . .
Jack and Annie find themselves on another adventure — this time they»re in medieval England. It»s exactly the same as the story book — there»s a castle and a banquet and knights on horseback. But when they find themselves locked in a dungeon, Jack starts to wonder if they»ll ever get home again . . .