bo jest ciekawa
Could this question also be understood as "What makes this book interesting?"?
The meaning is similar, but I would translate that as follows:
Co sprawia/powoduje, że ta książka jest ciekawa
Is "dlaczego myślisz, że ta książka jest ciekawa?" a suitable translation for "why do you think this book is interesting?"
'How is this book interesting?' would sound natural in Engish too. Would that also work in Polish: 'Jak książka jest ciekawa?'
Using 'how' instead of 'why' implies that you dont think the book is interesting, whereas 'why' is much more neutral. Not quite the same sentence.
That Polish sentence wouldn't work, but I added "how".