Translation:The book is strange.
I am not 100% certain but with the plural particle, AFAIK, it is more appropriate to suffix it to words that express persons or people, like person, professions, not objects like books, cars, I think in East Asian languages in general the concept of plural esp. for objects and animals is not as obvious as in Indo European languages
들 is like a plural emphasis marker. Things can be plural without it but if you need/want people to understand that there are more than one you use it. Also nouns in korean refer to the geniric idea of an object. Consider the difference between "the book is dirty" and "books are dirty" the sentence 책가 더럽습니다 can mean both "books are dirty" and "the book is dirty". Usualy this is fine because in context the meaning is usually clear. Hope understanding how nouns work can make the use of 들 make more sense.