Marcelo Cabuli is suing the author and publisher of Once Upon a Nightwish: The Official Biography 1996-2006 for libel. Proceedings began at Helsinki District Court on Tuesday.
The Argentine businessman -- husband and manager of the band's former lead singer Tarja Turunen -- is seeking more than 50,000 euros in damages over claims made in the book.
Three of his Brazilian business partners are seeking 15,000 euros each, bringing the total to nearly 100,000 euros. They argue that the book includes false accusations and insinuations that have caused them suffering and financial problems.
The book, which was published in Finnish in 2006 and in English three years later, blames Cabuli for the events leading up to Turunen's dramatic expulsion from the band in late 2005.
The mega-successful metal band is about to release its seventh album, its second with new vocalist Anette Olzon.
The group's record label claims that it is the most successful Finnish band internationally, with over seven million records sold globally.