In his debut thriller, Tolkien treads the literary path of his grandfather; like Lord of the Rings, Witness is about an underdog hero with a grave task -- young Thomas Robinson, who's determined to unmask the murderer of his monied mother, Lady Anne. It features a ring of evil power, a sapphire that Tom believes aided his mother's ruin. And it has typically Tolkienesque lines like ''His face was crushed into the soft center of her breast....'' -- well, you get the picture. J.R.R.'s grandson has written a potboiler brimming with secret rooms, secret drawers, and secret marriages. Not Middle-earth, but certainly middlebrow.