Daniel Domscheit-Berg, a cryptic computer scientist, was WikiLeaks' best-known face after Assange himself. Perhaps in true WikiLeaks style, his insider account is being published later this week simultaneously in 16 countries.
Publishers around the world have promised that Domscheit-Berg's "Inside WikiLeaks: My Time with Julian Assange at the World's Most Dangerous Website" will "tell the full story" and reveal never-disclosed details about the group's operations.
They said the book will explore WikiLeaks' evolution, finances and controversies - as well as the antagonism between Domscheit-Berg and Assange. The former spokesman left the project in September, citing personal, ethical and political differences with Assange.
Domscheit-Berg told The Associated Press in a telephone interview Monday that he wants to "show both the good and the bad sides of WikiLeaks, also when it comes to my relationship with Assange - and I tried to do this in a balanced way."
WikiLeaks spokesman Kristinn Hrafnsson was not immediately available for comment.
The book will be released in Germany, Australia, South Korea, Britain and 12 other European countries on Friday, according to its German publisher Econ Verlag. In the United States, it will be published four days later, on Feb. 15. Other countries including Japan, Brazil, and Russia plan publications soon.
Last month, Domscheit-Berg, 32, also launched a rival website called OpenLeaks, saying he planned to give whistleblowers more control over the secrets they spill. He has criticized WikiLeaks for both receiving documents and aggressively vetting how they are presented to the public.
"It's too much responsibility and too much power," he told reporters last week, when he announced the OpenLeaks launch at the sidelines the World Economic Forum in the Swiss resort of Davos.
While he has publicly called for more transparency at WikiLeaks and campaigned for freedom of information on the Internet, Domscheit-Berg has been very private when it comes to his personal life.
Some scant details come from news reports and Econ Verlag.
The man formerly known as Daniel Schmitt studied applied computer sciences in the city of Mannheim in southwestern Germany and was in charge of IT security at German branches of various international companies before he moved to Berlin to focus exclusively on his work for WikiLeaks.
He is married to Anke Domscheit-Berg, a manager at Microsoft Germany.
Often wearing black clothes and sporting a dark beard, Domscheit-Berg comes across as serious and thoughtful in public appearances. It's a contrast to Assange, often described as mercurial and hyper-kinetic.
After Domscheit-Berg quit his job at WikiLeaks, he told German news magazine Der Spiegel that Assange did not allow any kind of criticism regarding his person or the organization's work.
"Julian Assange reacted to any kind of criticism with the accusation that I was not obedient and not loyal regarding our project," he told Spiegel.
Assange told a London audience back in September that the German had been suspended - although he declined to go into details. He denied there had been a dispute over his management. "It was about a different issue," Assange said. He refused to elaborate.
Several reporters from The Guardian, The New York Times and Der Spiegel, who were involved in their news outlets' publication of leaked WikiLeaks documents, have also written books on their dealings with the group and Assange that will be published soon.
WikiLeaks has struggled to wade through the vast amounts of material it received - particularly hundreds of thousands of U.S. diplomatic cables - and has been criticized for sharing the data with only a handful of media outlets around the world.
Assange recently said he was making an effort to reach beyond the major newspapers - such as The New York Times and The Guardian - that worked with him on earlier releases, saying he hopes to enlist as many as 60 media partners.
The U.S. government has accused WikiLeaks of reckless disregard in the way it publishes documents, but Assange has repeatedly said he has done nothing wrong in releasing secret U.S. documents.
Raphael Satter in London contributed reporting to this article.
(This version CORRECTS Corrects that the book will appear later this week.)