This is the 2nd most frequent Turkish word.
için
"For" or "because of."
Here, 'için' is used to indicate purpose, showing for what reason the book is useful.
Bu kitap öğrenmek için çok faydalı.
This book is very useful for learning.
In this sentence, 'için' expresses intention, specifying the reason or purpose of buying the notebook.
Ders için yeni bir defter aldım.
I bought a new notebook for the class.
Lastly, 'için' denotes direction, specifying for whom or for what occasion the item is intended.