Imagine that you are Jeff Bezos. For four hours two weeks ago, you were the richest person in the world. And though Wall Street knocked you down a notch, pretty much everyone thinks it’s inevitable that you’re going to be number one again. You’re starting to be aware of the smell of the tar pits and you’re casting about for a way to put all that loot to some good. You're eying the Gates-Buffet Giving Pledge and thinking that if you donate half your fortune it should make a difference. You're comfortable with making older but meaningful institutions great again.

So far, you’ve concentrated on things that might benefit our distant successors—space travel, cancer treatments, AI, and a clock that will keep running for 10,000 years. But you want to do something more immediate. You say you want your philanthropic activity “to be helping people in the here and now—short term—at the intersection of urgent need and lasting impact.” You are open to suggestions--so much so that you even recently tweeted a “request for ideas.”

Though you don’t mention it, I suspect you’re thinking of stepping into an area that traditionally government might have addressed—but now, in an era in which the wealthy are doing better and better, benefits seem to go toward the top while the “urgent needs” of just plain people are left to the grace of a harsh marketplace. Like it or not, citizens are increasingly dependent on the kindness of strangers with billions of dollars,

I have a suggestion for you, Jeff Bezos. How would you like to become the Andrew Carnegie of our time?

Yes, I am talking about libraries. Those places where books sit on shelves, not delivered by FedEx. And so much more. Carnegie made them the center of his philanthropy, and almost became synonymous with them. More importantly, he changed countless lives with his investments in libraries. I have heard that you’re looking for big ideas, and this is one.

Today, local libraries are thought of as slightly retro public institutions. For some reason, major donors don't get excited about them. OK, there are some notable exceptions to this rule—in my adopted city of New York, for instance, Stephen Schwarzman has his name engraved on the main branch building of the public library; in Kansas City, the Kemper family has donated millions to the downtown branch and a Kemper scion, R. Crosby Kemper III, has been the executive director of the library for more than 12 years.

But the real impact—the one that changes lives and transforms communities—has yet to be made. It turns out that libraries are the very model of the more-than-shovel-ready, here-and-now, urgent-need-and-lasting-impact places that you as a tech philanthropist claim to be interested in supporting in a big way. And libraries’ needs are dire.

You, Mr. Bezos, may not have been inside a library in a while. Things have changed. Today, libraries are serving as essential civic places. Trusted by every part of American society, they're the only noncommercial places other than city squares where people meet across genders and ages. They provide all kinds of services and programming—just visit the glorious Madison, WI Central Library, where a first-rate makerspace is under the same LEED-certified roof as local service agencies helping people sign up for health care and food assistance.

Librarians are not shushing people, and libraries are no longer only silent cathedrals for solo reading. (They still have reading rooms—don't worry.) Instead, these great pieces of civic architecture are being repurposed: They're places that offer classes in computer skills and thousands of other subjects, provide internet access to millions of Americans who can't afford it, and host innumerable neighborhood meetings.

Libraries these days are providing meals to kids and adults through local food banks, working with local immigrant agencies, offering homework help, and loaning out an amazing array of things, from musical instruments to microscopes. (Yes: the Library of Things.) What they're up to is dazzling. And in 2013, 94 percent of Americans said that having a public library improves the quality of life in a community. As America gets older and more unequal, its people need new forms of education to thrive—and libraries are ground zero for every public value the country cares about.

The American Library Association says that America’s more than 120,000 public, school, academic, and special libraries are visited more than 1.4 billion times a year by hundreds of millions of Americans in every corner of the nation and from every walk of life. They complement but do not compete with your mighty commercial bookselling venture, Mr. Bezos. At the same time, libraries are chronically under-resourced. Limited hours. Limited staff. Low pay. Constant need for renovation. Overcrowding.

Libraries are attempting to serve people in an era of thin government support, increasing need, and staggering inequality—much like the era that gave us Andrew Carnegie. His response to the problems of his time was to build thousands of public libraries across the country, starting in 1886. Most of those beloved community libraries are still functioning. Carnegie aimed high, wanting to make the world better than he found it. And he succeeded.

Here's the twist in the story that you, Mr. Bezos, may not know: Carnegie's money was given on the condition that local public authorities step up with pledges to support and maintain the institutions that he launched. For Carnegie, this structure fit with the idea that communities were being helped to help themselves—a pillar for him. Many cities turned down Carnegie's offer, and later regretted it.

If you are looking to have your name be kept alive in the memories of generations—or if you simply want a legacy worthy of the fortune you have reaped—you don't need to start something new or even have it named after you. (You didn't rename the Washington Post, either, and yet it's becoming one of the handful of great news sources in the world.) Hidden in plain sight, the local libraries of America are patiently waiting for your attention. (They're also often really beautiful spaces, and I can tell that you like design. Just down the street from your headquarters is Rem Koolhaas’s terrific Seattle main library, with areas named after donors and relatives of Paul Allen, Microsoft, Charles Simonyi, and Boeing.)

Whether or not the local library a random American uses today was actually built by Carnegie, he or she knows what that philanthropist did. More important, if a philanthropist was someone who wanted to get a glimpse of what his money did, he would be proud of what his money had accomplished.

Tragically, the federal government and the states are constantly cutting back on library funding. You would almost think that politicians don’t want members of the public to have access to the very knowledge that would lead them to make informed decisions! But those politicians are ignoring the fact libraries are citadels of civilization and economic ladders for those otherwise stuck on the bottom rungs. Why not use the lever of your money, Mr. Bezos, to spur public authorities to do their part? Just like Carnegie did. It is hard to imagine a better use of billions.