The Pulitzer Prize committees responsible for awarding journalists and newspapers have often been condemned for their liberal bias. Columbia University, the Prizes' sponsor, makes no bones about the "liberal heritage" of the awards. Named after Polish-American journalist Joseph Pulitzer (whom many accused of turning serious journalism into an entertainment-oriented free-for-all), the annual award goes to one recipient in each of 21 categories ranging from local reporting to music to short stories and poetry.
If the journalism categories are slanted too far left, the category for novels offers something of a balance. While there are plenty of liberals represented, there are also plenty of moderates and conservatives, and the prize is generally awarded to genuinely deserving authors. Each year, one writer gets $10,000 for a work evidencing fine craftsmanship, universal themes, and (generally) characters or situations characteristic of the American experience. Authors must be American, and their work must specifically be submitted to the judging committee for consideration.
Some of the best American novels of the last hundred years have won the Pulitzer Prize, though many excellent works have also been neglected. An award like this one is necessarily susceptible to the whims and accidents of human judgement and error, and though the committees are large, they're also collectively imperfect. Don't use this as an absolute guide for must-read American novels; instead, use it as a general reference, and you may be surprised by some of the little-known gems you discover.