From the site:
In compiling the books on this list, the editors at SuperScholar have tried to provide a window into the culture of the last 50 years. Ideally, if you read every book on this list, you will know how we got to where we are today. Not all the books on this list are “great.” The criterion for inclusion was not greatness but INFLUENCE. All the books on this list have been enormously influential.
The books we chose required some hard choices. Because influence tends to be measured in years rather than months, it’s much easier to put older books (published in the 60s and 70s) on such a list than more recent books (published in the last decade). Older books have had more time to prove themselves. Selecting the more recent books required more guesswork, betting on which would prove influential in the long run.
We also tried to keep a balance between books that everyone buys and hardly anyone reads versus books that, though not widely bought and read, are deeply transformative. The Grateful Dead and Frank Zappa never sold as many records as some of the “one-hit wonders,” but their music has transformed the industry. Influence and popularity sometimes don’t go together. We’ve tried to reflect this in our list.