Mr Verschave is a member of a pressure group which aims to expose corrupt African regimes and the role which successive French administrations played in supporting them.
The presidents of Congo-Brazzaville, Chad and Gabon brought the case against Francois-Xavier Verschave under a law from 1881 which makes it a crime to offend a foreign head of state.
In the book Noir Silence (Black Silence), the author accuses the leaders of everything from drug trafficking to mass murder.
But the court dismissed their claim, declaring that the law was in contravention of the European Convention on Human Rights, which protects freedom of speech.
Corruption exposed
The court said there was no legal definition of "offend" under the terms of the law and the term was so broad as to be almost impossible to define.
Mr Verschave is a member of a pressure group which aims to expose corrupt African regimes and the role which successive French administrations played in supporting them.
French backing for African states is a theme which has been evoked in other recent high-profile cases here - one involving the former state-owned oil company Elf, and another focusing on Jean-Christophe Mitterrand, the son of the former French president.
An earlier book by Mr Verschave called France Afrique also proved controversial, although a libel suit brought by the former French Interior Minister, Charles Pasqua, only resulted in the author paying symbolic damages of a single franc.
The African leaders have not sued for libel.