"He was driven to write 12 hours a day for three years," she said. "It began as a diary. Then he thought he would write a medical paper; then he realized that medical science could not explain it all."
"Proof of Heaven" debuted at the top of The New York Times bestseller list and has sold 1.6 million copies, according to its publisher.
Alexander says he didn't know how to deal with his otherworldly journey at first.
"I was my own worst skeptic," he said. "I spent an immense amount of time trying to come up with ways my brain might have done this."
Conventional medical science says consciousness is rooted in the brain, Alexander says. His medical records indicated that his neocortex --- the part of the brain that controls thought, emotion and language --- had ceased functioning while he was in a coma.
Alexander says his neocortex was "offline" and his brain "wasn't working at all" during his coma. Yet he says he reasoned, experienced emotions, embarked on a journey --- and saw heaven.
"Those implications are tremendous beyond description," Alexander wrote. "My experience showed me that the death of the body and the brain are not the end of consciousness; that human experience continues beyond the grave. More important, it continues under the gaze of a God who loves and cares about each one of us."
Skeptics say Alexander's experience can be explained by science, not the supernatural.
They cite experiments where neurologists in Switzerland induced out-of-body experiences in a woman suffering from epilepsy through electrical stimulation of the right side of her brain.
Michael Shermer, founder and publisher of Skeptic magazine, says the U.S. Navy also conducted studies with pilots that reproduced near-death experiences. Pilots would often black out temporarily when their brains were deprived of oxygen during training, he says.
These pilots didn't go to heaven, but they often reported seeing a bright light at the end of a tunnel, a floating sensation and euphoria when they returned to consciousness, Shermer says.
"Whatever experiences these people have is actually in their brain. It's not out there in heaven," Shermer said.
Some people who claim to see heaven after dying didn't really die, says Shermer, author of "Why People Believe Weird Things."
"They're called near-death experiences for a reason: They're near death but not dead," Shermer said. "In that fuzzy state, it's not dissimilar to being asleep and awakened where people have all sorts of transitory experiences that seem very real."
The boy who saw Jesus
Skeptics may scoff at a story like Alexander's, but their popularity has made a believer out of another group: the evangelical publishing industry.
While the church may be reluctant to talk about heaven, publishers have become true believers. The sales figures for books on heaven are divine: Don Piper's "90 Minutes in Heaven" has sold 5 million copies. And "Heaven is for Real: A Little Boy's Astounding Story of His Trip to Heaven and Back" is the latest publishing juggernaut.
"Heaven is for Real" has been on The New York Times bestseller list for 126 consecutive weeks and sold 8 million copies, according to its publisher.
The story is told from the perspective of Colton Burpo, who was just 4 when he slipped into unconsciousness while undergoing emergency surgery for a burst appendix.
Colton says he floated above his body during the operation and soared to heaven, where he met Jesus. Todd Burpo, Colton's father, says he was skeptical about his son's story until his son described meeting a great-grandfather and a miscarried baby sister --- something no one had ever told him about.
Todd Burpo is a pastor, but he says he avoided preaching about heaven because he didn't know enough about the subject.
"It's pretty awkward," he said. "Here I am the pastor, but I'm not the teacher on the subject. My son is teaching me."
Colton is now 13 and says he still remembers meeting Jesus in heaven.
"He had brown hair, a brown beard to match and a smile brighter than any smile I've ever seen,'' he said. "His eyes were sea-blue, and they were just, wow."
Colton says he's surprised by the success of his book, which has been translated into 35 languages. There's talk of a movie, too.