Valérie Trierweiler, the president's 47-year-old partner, is suing the authors of "La Frondeuse" (The Rebel) for defamation and invasion of privacy, seeking 85,000 euros in damages and court costs, reports RFI.
On Monday, it emerged that Hollande had sent a letter to the court, denying an assertion in the book that he had reached out to the Right in the mid-1990s.
Hollande denounced as a "complete fabrication" the book's claim that he had sent a letter in 1994 to former right-wing prime minister Edouard Balladur.
"The separation of powers has been violated. The president is the guarantor of judicial independence. This is unbelievable," said Olivier Pardo, the lawyer for the book's two authors.
The right-wing opposition UMP also expressed its "amazement" at the letter, which it said "can legitimately be considered a means of putting pressure" on the court.
The letter mentioned in the book was allegedly given to former right-wing minister Patrick Devedjian.
The book, written by television journalists Christophe Jakubyszyn and Alix Bouilhaguet, claims Trierweiler had an affair with Devedjian early on in her relationship with Hollande.
Read more of this report from RFI.