The most thoughtful and provocative book on photography I’ve read in a long time contains not a single photograph, but it’s full of memorable images. For Photographs Not Taken (Daylight), editor Will Steacy asked 62 photographers (including Emmet Gowin, Mary Ellen Mark, Tim Hetherington, Alec Soth, and Zwelethu Mthethwa) to describe the ones that got away—the “mental negatives” that haunt them years later. The results—brief essays, many no more than a page—are unexpectedly eloquent and revealing. For a lot of the respondents, the missed photograph involves no sense of regret, only the realization that some moments are too ephemeral to frame and preserve. When his equipment was too far out of reach to capture the image of his young son’s spontaneous communion with a smoke-screened sunburst, Chris Jordan says, “my eyes were the camera and my memory is the print.” But for others, the decision to put the camera aside is a fraught and painful one. Elinor Carucci, who has never shied away from recording intimate situations, found herself torn when she gave birth to twins: “Every moment, I had to choose between photographing and mothering, and when I did choose photography, every photograph became a second of guilt, a second I neglected them, a second I thought about light, composition, exposure, and not about them.” When, as she puts it bluntly, “the photographer declared war on the mother,” the mother usually won, but the images Carucci missed remain “stored in [her] eyes.”

For many photographers, ethical questions loom especially large. The camera allows for a special kind of access, but there are limits to the protection and permission it grants. After befriending a jazz musician down on his luck, Doug DuBois made pictures of him at his most vulnerable that he regrets more than any he may have missed. “Putting the camera down is just as hard as picking it up,” he confesses, “and…my path to an ethical and honest understanding of what photographing means, as opposed to what photographs mean, is marked by occasional lapses of discretion and decency.” Sylvia Plachy headed to Lower Manhattan on September 11, 2001, but, confronted with a man covered in plaster dust, she couldn’t bring herself to step in front of him and raise her camera. “I felt ashamed, I hesitated, I questioned,” she writes. “It didn’t seem right.” Joshua Lutz was in the city that day, too, but found himself similarly frozen. “It’s not that I didn’t want to take a photograph,” he writes. “I couldn’t. I was no longer a part of my body.” He says something echoed by many of the other photographers here: “I have used photography to separate myself from some pretty horrific experiences in my life. Being on one side of the lens and not the other, I was able to turn on and turn off my relationship to these experiences…As much as it can help to separate myself from difficult situations, it can also keep me from truly experiencing events.”

These serious concerns resonate throughout Photographs Not Taken, but the mood is far from dark, primarily because each photographer provides at least one wonderfully vivid image in words (Aaron Schuman: “Six horses galloping through dawn mist, outside the window of a speeding train;” Alessandra Sanguinetti: “Sunset, gray light, 1978, my father’s carrying a transparent plastic bag with our dead dog to its grave under the willow tree”), a useful maxim or two (Erika Larsen: “The photographs not taken are the photographs not given”), and plenty of good advice (Massimo Vitali: “When you see something, you should shoot it immediately, never thinking that tomorrow could be better, because it will almost always be worse”).