The lawsuit "had a negative impact on Relin's livelihood as an author," his attorney said in an August 2011 court filing.
Relin didn't maintain any insurance and had to personally fund his defense.
In an introduction that Relin wrote for "Three Cups of Tea," he acknowledged potential inaccuracies, wrote attorney Sonia Montalbano in another filing. Mortenson's "fluid sense of time made pinning down the exact sequence of many events in this book almost impossible," she wrote
Relin was not involved in a separate investigation by the Montana attorney general into how Mortenson ran the charity, which led to a settlement in April that called for Mortenson to reimburse the charity nearly $1 million.
Conflict between the co-authors was evidenced in a 2008 interview with the University of Oregon literary journal Etude.
Relin said he had objected to Mortenson being identified as co-author.
"That's been the only negative thing about this whole adventure for me," Relin said.
"It was published that way over my objections," he added.
Mortenson did not respond to phone calls from The Associated Press seeking comment.
The publisher released a statement saying, "All of us at Penguin are saddened to hear of the death of David Oliver Relin. We extend our heartfelt condolences to his family."
Relin's second book, "Second Suns, Two Doctors and Their Amazing Quest to Restore Sight and Save Lives," was scheduled to be published in June.
Relin was born in Rochester, N.Y. He is survived by his wife, two sisters, and his mother.
AP writer Matt Volz contributed to this report from Helena, Mont.