This is the 4th most frequent Catalan word.
en
"En" in Catalan is a weak pronoun meaning "of it," "of them," or "some," often replacing phrases introduced by "de."
In this sentence, 'en' is a courtesy title used before male names in Catalan as an honorific marker.
Vaig conèixer en Joan ahir.
I met Joan yesterday.
Here, 'en' refers back to the complement 'un llibre,' acting as a pronoun.
Va trobar un llibre i en va parlar molt.
He found a book and talked a lot about it.
In this sentence, 'en' is part of the prepositional phrase 'en aquell moment,' indicating a specific time.