"That book is especially interesting."

Translation: 그 책이 특히 재미있어요.

[tessabanessa]

I feel like the topic particle would fit nicely with 특히 in stressing that this book in paticular is especially interesting.

[jlseymour3]

I agree, reported 1/22/18.

[Kyu_99]

책은 is grammatically correct, because it's an object, right?

[oee16]

(1) Yes. 책 is an "object" in the sense of being a "noun".

But in this sentence, it is not an "object complement" (usually marked by 를/을).

(2) re. That book

그 책이 = The book there (identification)

그 책은 = The book referred to (categorization)

[Shaxton]

An I right that you mean 그 책은 might be used to mean something like "the book we were just talking about" and 그 책이 would be used to refer to a book mentioned for the first time?

[oee16]

Correct.

이/가 => identification marker, used to index a potential topic of interest

은/는 => narrative marker (similar to re./wrt), used to recall a topic that has already been marked 'of interest'.