The sentence refers to the book as having some kind of correct information. Rather than saying some book out of several is the correct one.
I think the particle 이 after book is the subject marking particle which puts the emphasis on book.
I wrote "That book's right", and it was marked as wrong. Am I missing something?
Please do not contract is to 's after a common noun; Duolingo accepts it only after pronouns such as he's for he is.
"-들" would make 책 plural, but because it is not present in this sentence, it's natural to assume there is only one book.