These are the books by Katherine Langrish that are reviewed on this site:
Down, but not out, Peer Ulfsson survives his ghastly uncles, cold, hunger and loneliness, only to face the Troll King. He could save himself, but goes back for his friend...
Peer Ulfsson, fighting to find his own place in the world, learns that everything actually has to come second to loyalty and love for friends and family. Plenty of trolls and strong magic.