A few I found helpful:
The Orthodox Way by Fr. Timothy Ware (Discusses doctrine, practices, worship, etc. of the church)
The Orthodox Church by Fr. Timothy Ware (Mostly about the history and basic doctrine of the church; a little dry in places.)
Becoming Orthodox by Fr. Peter Gillquist (Personal account of conversion to Orthodoxy, especially good if your friend is coming from a Protestant background)
Facing East by Frederica Mathewes-Green (Sort of a personal diary of her experiences throughout the church year; I found this helpful with understanding feasts, fasts, and such.)
Great Lent by Fr. Alexander Schmemann (Great for a better understanding of fasting, the Canon of St. Andrew, and other Lenten traditions)