كِتَابُ سِيبَوَيْهِ
Sibawayh’s book.
سَفِينَةُ نُوحٍ
Noah’s ark.
A noun annexed to a definite expression can become identifiable through that relationship. «كتاب سيبويه» identifies the book by its relation to Sibawayh; «سفينة نوح» identifies the ark through Noah.
An idafa need not mean simple ownership. It can express relationship, association, distinction, respect, and other contextual meanings.