This is the 9613th most frequent Spanish word.
aquél
"That one" (masculine, singular, used to refer to something far away in distance or time).
In this case, 'aquél' is a pronoun replacing 'ese libro' to avoid repetition.
Aquél es mi libro favorito.
That is my favorite book.
Here, 'aquél' serves as a demonstrative pronoun referencing a specific item among others.
Prefiero aquél que tiene tapas duras.
I prefer that one that has hard covers.
'Aquél' indicates a specific individual or object already known by the context.