The passive voice will be A book about a legend is bought by Norin.
In the active voice, the sentence "Norin buys a book about a legend" has the subject (Norin) performing the action (buys) on the object (a book about a legend).
To convert this sentence into passive voice, you switch the order of the subject and the object, and you also change the verb form. In passive voice, the object of the active sentence becomes the subject of the passive sentence, and the verb is changed to a form of "to be" plus the past participle of the main verb.
So, in the passive voice, "A book about a legend is bought by Norin," the focus is on the book rather than Norin. The action of buying is still conveyed, but the structure of the sentence has changed to highlight the receiver of the action (the book).