I think it's better to use it in a singular form as it shows the level on a scale to show the quality of the book. However, if you see the book in a list among other book titles, it's better to use it as a plural noun.
7 november 2019
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Heb je je antwoorden nog steeds niet gevonden?
Schrijf je vragen op en laat de moedertaalsprekers je helpen!