This is the 10820th most frequent Russian word.
выдавшего
"Выдавшего" means "having issued" or "having given out."
Here, 'выдавшего' is the singular masculine genitive participle form of the verb 'выдать', used to refer to someone who committed the act of revealing something in the genitive case.
Он спросил о человеке, выдавшего секрет.
He asked about the person who revealed the secret.
In this sentence, 'выдавшего' refers to someone (the author) who brought or published an amazing book, functioning as a singular masculine accusative participle.
Мы узнали автора, выдавшего замечательную книгу.
We found out about the author who brought out an amazing book.
Here 'выдавшего' describes the person who betrayed or gave someone away, and it is used to complement 'поступок' (action) in a possessive context.