When Rupert Isaacson decided to take his autistic son, Rowan, on a trip to Mongolia to ride horses and seek the help of shamans two years ago, he had a gut instinct that the adventure would have a healing effect on the boy. Mr. Isaacson’s instinct was rewarded after the trip, when some of Rowan’s worst behavioral issues, including wild temper tantrums, all but disappeared.

Now the publisher of Mr. Isaacson’s book about the journey, “The Horse Boy,” has a similar instinct about the market potential of his story, and is hoping for its own happy ending.

Little, Brown & Company, which released “The Horse Boy” on Tuesday, has a lot riding on its success: the publisher paid more than $1 million in an advance to Mr. Isaacson before he and his family had even taken their Mongolian trip.

Michael Pietsch, publisher of Little, Brown, said booksellers had already placed orders high enough to justify a first printing of 150,000 copies.