This is the 130th most frequent Croatian word.
kojeg
"Which" or "whom" (masculine singular genitive or accusative form).
Here, 'kojeg' corresponds to 'the book' (masculine gender, genitive case) introduced by the preposition 'zbog' and serves to link the reason with the book.
Knjiga zbog kojeg sam ovdje je vrlo zanimljiva.
The book because of which I am here is very interesting.
The word 'kojeg' refers to the dog (masculine gender, accusative case) and is used in a relative clause to specify which dog is being referred to.
Ovo je pas kojeg sam vidio jučer.
This is the dog that I saw yesterday.
In this example, 'kojeg' refers to the boy (masculine gender, accusative case) who is the object of the action in the relative clause.