This is the 6894th most frequent Turkish word.
böylesine
"To this extent" or "in such a way."
Here, 'böylesine' is used to emphasize the extremity or particular nature of the beauty he is referring to, equivalent to 'such a.'
Böylesine güzel bir günü uzun zamandır görmemiştim.
I haven't seen such a beautiful day in a long time.
In this context, 'böylesine' highlights the uniqueness or specific interesting attribute of the book mentioned.
Böylesine ilginç bir kitabı herkes okumalı.
Everyone should read such an interesting book.
In this sentence, 'böylesine' points to the specific difficulty or extremity of the situation being described.