This is the 6404th most frequent Portuguese word.
didier
"Didier" is not a Portuguese word; it is typically a French given name.
In this sentence, 'Didier' is used as a proper noun, serving as the name of a person whose book is being described.
O livro de Didier é muito interessante.
Didier's book is very interesting.
Here, 'Didier' is again a proper noun, indicating the name of an individual with the title 'Professor' associated with it.
A palestra foi conduzida pelo renomado Professor Didier.
The lecture was conducted by the renowned Professor Didier.
In this third example, 'Didier' is the proper noun identifying the author of the article described.