This teenage son still hasn't forgiven his mother for her affair when he was ten years old. So, when she married the man she had been cheating with, he gave her a book on cheating for their anniversary. This week, Reddit user u/IGaveTheBook turned to the r/AmItheAsshole subreddit seeking advice and insight on his actions. Contributors often post to this subreddit for third-party insight into their issues, asking readers and commenters whether or not they are the a-hole in the situation. u/IGave…