A book that I used to teach trustworthiness and honesty is Eli's Lie-O-Meter. This book teaches that lies have consequences not just for the liar, but for those around them as well.
After reading the story we make a list of each lie that was told in the story. We then talk about honest strategies that could have been used. I make each student a "truth ticket" like the one in the story as a reminder to tell the truth.