This is the 7540th most frequent Turkish word.
hakkındaki
"About" or "regarding."
Here, 'hakkında' is used to indicate the subject or topic of the book, i.e., historical places in the city.
Bu kitap şehirdeki tarihi yerler hakkındadır.
This book is about the historical places in the city.
'Hakkındaki' is used to express opinions or thoughts concerning a person, specifically the friend in this case.
Arkadaşının hakkındaki düşüncelerini öğrenmek istiyor.
He wants to learn the thoughts about his friend.
In this sentence, 'hakkındaki' specifies the topic of the director's views, which is art.