Are the books I bought considered forbidden (haram) because I cut in line ahead of someone, and how can I atone for this sin?

Cutting in line is an act of aggression and injustice that is impermissible, and its perpetrator incurs sin. However, it does not affect the validity of buying books because the prohibition here is due to an external matter unrelated to the transaction of purchase itself. The sin can be expiated by seeking forgiveness from the person who was trespassed upon, if possible, or by seeking Allah's forgiveness (istighfar) and repentance.