|In addition to Atlas Shrugged, I voted for The Federalist, and Riders of the Purple Sage.|
Several others, of course, could have ranked as highly. Thomas Paine's Common Sense was read to the troops at Valley Forge. It was reprinted often - much to Paine's own chagrin, as he received no income from the pirated editions.
I believe that Walden, in particular, became popular only in our time because it was assigned by teachers and professors seeking to raise questions about industrial society. So, Walden was influential, as well, though it was largely ignored in its own time.
Zane Grey's book did reflect American values, more than shape them, but I credit it with validating the dime novel, not so much for literature teachers and professors, but for the readers and writers who made popular fiction - from westerns and detectives to science fiction and comic book superheroes, including Philip Jose Farmer's "Riders of the Purple Wage" which appeared on its own, but is best known for being included in Harlan Ellison's "Dangerous Visions" attempt at post-modernist science fiction.