Last in the Katherine Langrish Troll Trilogy, ''Troll Blood'' follows Peer and Hilde on a voyage even more dark and dangerous than their previous, troll-fighting adventures. When a Viking longboat lands at their village, Hilde is desperate to join its crew. The Water Snake is headed for Vinland, recently discovered by Leif Eriksson. When Peer agrees to accompany her, Hilde's parents reluctantly agree.