The Jungle by Upton Sinclair certainly put a spot-light on the meat packing industry and made the country question their food supply and its sanitation practices.

Great discussion topic, and great answers. I would add Silent Spring by Rachel Carson for its role in encouraging the environmentalist movement. Two others might be De Revolutionibus by Copernicus and Principia Mathematica by Isaac Newton for obvious reasons. 

Outside of the novels which have been mentioned, I would add Common Sense by Thomas Paine, Divine Comedy by Dante Alighieri, and Harry Potter by JK Rowling. I am sure that I do not have to explain the first two, but the final suggestion may seem a little confusing. As a parent and teacher, the Harry Potter series ignited a fire for reading under many young and more mature readers alike. As far as I can remember, no other book has gotten so many people to read.

I would select Fielding's "Tom Jones' as strategically important to the development and success of the novel form. I also think 'To Kill a Mockingbird' has had a tremendous impact internationally.

I would suggest that Paradise Lost is one of the books that changed the world, because it so strongly influences how we view Heaven, Hell, morality and life.  Along those lines, I would also suggest some of the works of Shakespeare, Dickens, Twain and Steinbeck for having influences in their own countries in terms of social justice and views.

The Bible. It is still influencing people today, 2000 years after most it's historical references occurred.

The Origin of Species, which started the whole evolution debate that is still going on today.

Mein Kampf, which preceeded Hitler's rise to power.

One might be the Koran.  It's not so much the book itself as the message of Islam, but the rise of Islam surely changed the world.  

A second would be The Communist Manifesto.  By inspiring communism, Marx and Engels created the system that would help lead to the Cold War that dominated the second half of the 20th century.

The first book that comes to mind is one that often gets credit for helping to end slavery in the United States - Uncle Tom's Cabin. 

Though the actual impact of the book is difficult to measure (and may be exaggerated to some extent), this novel is credited with helping to popularize the abolitionist movement by humanizing all the figures in the debate/slavery system.