 When Booth asks him why, he shows Booth the letter, explaining that he chose to ask Booth as the latter "[knows] more about duty and honor than anyone else I know".Before Addy goes to Iraq, Booth gives him a harmonica as a parting gift.Following the climax of the events at the end of season 3, Booth regrets that he had never talked much to Zack, feeling that if he had he would have caught onto Zack's affiliation with Gormogon sooner and could, perhaps, have done something about it.