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 Kasım 2019
0
1
Hâlâ cevap bulamadın mı?
Sorularını yaz ve ana dil konuşanlar sana yardım etsin!