The Communist Manifesto by Karl Marx and Frederick Engels

The Communist Manifesto details the viewpoint of the Communist Party of 1848.  Many people at the time had misconceptions about both the standpoint and the reasons behind it.  There were also a lot of common arguments that the manifesto tried to dispel. 

I personally love to read about other people's philosophies, so this was a fun read for me.  Even though the philosophy in question has caused so much trouble in our world, it was still fun to read about.  If you're looking for books with the most historical significance, this would probably also make the top ten.