'The Black Book' Bump.
Richard Pipes "Communism" is a very short and excellent primer on the subject. Dr. Pipes is a Harvard historian, but his tome is not at all sympathetic, proving perhaps that not all legitimate scholars have been cleansed from Cambridge yet.
One point he makes in the introduction is that it is not possible to discern a significant difference between "Socialism" and "Communism", they are one and the same.
This is an excellent book to give to liberal friends.