My latest book, What Matters, contains 18 long-term photo essays about essential issues of our time by some of the great photojournalists of this generation. Some of them — Nachtwey, Salgado — are household names; others aren't — at least not yet — but all of them are masters of their craft and deeply passionate about their subjects. How passionate? Passionate enough to shoot, say, water issues in 30 countries for five years… or malaria on three continents for two years… or AIDS in sub-Saharan Africa for a decade, all under tough conditions, most often with little pay. Imagine having that much passion, having such a powerful inner need to tell the stories that need telling no matter the obstacles.

I worked on What Matters for 18 months, developing story ideas, gathering photographs and commissioning top commentators such as Jeff Sachs at Columbia University and Pulitzer Prize winner Samantha Power at Harvard to write essays that would explain and elucidate the picture stories. I added a comprehensive "What You Can Do" section for readers who wanted to convert outrage into action — just in case that happened.

What Matters was finally published in September 2008, and since then I have done scores of media interviews in hopes of "getting the book out there," as they say. This sort of shilling is necessary because the fact is most people won't voluntarily shell out 20 bucks for a book that shoves tough issues in their faces, and I need to reach those few who will. I've done as many as 18 radio interviews in one day, and eventually during the course of every single interview, the questioner inevitably asks me the same question: "Why did you decide to do this book?"

When you know that you will be asked the same question 100 times, you have a choice: you can either give a stock answer, a 15- to 30-second sound bite that sounds smart and might move some books… or you can think about the answer and refine your thoughts over time. With most of my books — Day in the Life and America 24/7 — I do the former. I have been doing this so long, the answers are burned into my cortex, but What Matters is a change of course for me, so I have been considering my words each time.

Don't get me wrong. My out-of-the-gate stock response was true: I walked into the sprawling new Barnes & Noble near my home outside San Francisco; I looked at all the lovely illustrated books there and saw all the usual subjects, all well done and glossy, and I decided on the spot to create a book about something that really mattered to me — namely how the language of photojournalism could enlighten readers about the crucial issues of our time; how photojournalism could capture a decisive moment and serve it up for consideration and actually make the viewer think about the meaning of that moment; how great photojournalism is always personal and specific, but then makes you think about the larger ramifications.

Not a bad response, and as I said, all true.

But after a while, I segued into a different answer, which is also true: When I started in this business of photojournalism 30 years ago, I worked for a great photo editor named Robert Pledge, at a great photo agency called Contact Press Images, with great photographers that included David Burnett, Eddie Adams, Alon Reininger, Dilip Mehta and Annie Leibovitz. At Contact, we consciously pursued stories that could make a difference, create an uproar and demand change. It was a meaningful time in my life and career, but after three years, I left Contact and hooked up with Contact photographer Rick Smolan on a book project that became A Day in the Life of Australia. And that became the Day in the Life series … 15 volumes that took me around the world … and then, over the course of many years, that became America 24/7 and lots of other big collaborative projects, some very successful, some not so much. With What Matters, I wanted to circle back to that time at the beginning of my career when we believed that photojournalism could make a difference, that photography could change the world.

Again, all true.

But then finally, I distilled the answer further. To wit: I created this book because I believe in my heart that one great photograph can change the world. And if I can show 250 great photographs about the crucial issues of our time to enough people, then maybe one of those people, or maybe a few people, or, maybe even many of those people will connect with an image. And when one great image resonates with one talented and dedicated person, and that person digs deeper, learns more and takes some action that creates positive change in the world, then What Matters can be considered a useful exercise. I can't predict who that person will be or which of the 250 images in What Matters will resonate, or what action that person will take, but I completely believe it will happen.

The proof of that distilled theory came very quickly, and not how I expected. Two months after What Matters was published, a friend e-mailed me a link to a Bill Moyers interview with someone I never heard of, a musician named Mark Johnson. (http://www.pbs.org/moyers/journal/10242008/watch3.html) Turns out, Mark had done some amazing things. He created a documentary film called "Playing for Change" in which musicians from all over the world – New Orleans, Italy, the Congo, South Africa – all played the same songs, and Mark mixed their distinctive musical styles together to create beautifully affecting medleys. More concretely, Mark also built a music school for kids in a poor township near Capetown, South Africa, a community center that brightens these kids' otherwise difficult lives. And that first school was such a success that he is now building similar schools all over the world. At one point in the interview, Bill Moyers asks Mark Johnson why he decided to do all of this good work. Here's Mark's answer, which I've shortened a bit:

Many years back, my brother [gave] me a Christmas gift… a photo book called "A Day in the Life of Africa." And in that book was one photograph… and the caption was something along the lines of, "One of the more dangerous townships in South Africa finds solace through backyard jazz." And I had this picture on my wall for years. And it served as a symbol for me and for the crew that I [was] traveling with.

I did some research. And I found out that the bandleader was the upright bass player named Pokei Klaas. When we traveled down to Cape Town, South Africa, we heard this music down the street. So the crew and I walked over to hear their music. And when the song was over, we asked Joe Peterson, who was the singer in the band, "Have you ever heard of Pokei?"

And he said, "Oh, yeah, Pokei. He's my best friend. I'll take you to see Pokei."

So the next day, we all got in a van and we drove out to Guguletu township. We passed thousands of shacks and [it was] an incredibly humbling experience. I remember there were a number of little homes, and a lot of sorrow because there was a lot of HIV in the area. A lot of poverty.

So we decided, okay, we'll put on a little concert in the backyard because the people here need something to celebrate.

And I have never in my life seen something more beautiful when the people came out of their little homes and just started dancing and celebrating this music. And all the sorrow was gone, and they were now filled with all this joy and connection to us and to each other. So we asked Pokei, "Well, what can we do to give back to your community?

And Pokei said, you know, "The kids here, they really need a music school. They need some hope. They need something that can give them some inspiration." And so just a couple months ago we went down there with some shovels and we built the first Playing for Change music school in that exact spot. In the backyard. And now it's a chance for kids to get together, to have something positive to look forward to. And what we're doing with this foundation is we're going to build hundreds of schools around the world.

So there it was – proof of concept. One person, Mark Johnson, connected with one photo shot by a black South African photographer named Fanie Jason in a book, A Day in the Life of Africa, that I created six years earlier. That photo hung on his wall for years, and eventually it inspired him to build music schools for disadvantaged children around the world.

So, like I said, I don't know which photograph in What Matters will connect with which talented, dedicated person like Mark Johnson, but it will happen, I promise, because I know in my heart that one photograph can change the world.