Thanks for accepting my very old suggestion of "What is the sequel of the book”. Now I look at it again, it sounds odd too and needs something that isn't in the Portuguese to work well, for example, "What is the sequel of the book called?" ("What's the book's sequel called?").
The current answer "What is the book's continuation?" sounds even more odd, and seems to be trying to say "How does the book continue/end". I'm struggling to find a use of the word "continuation" that sounds totally natural here.
What is the continuation of the story? You might ask that when someone is telling a story, gets interrupted, and moves to another topic, but you want to hear the end of the story.
But more naturally you'd say Continue the story, what's the end(ing)? or something of that ilk.
My answer, "What is the continuation of the book?", was marked correct. I entered this because by now I know that a weird literal translation stands a better chance of being marked correct than a more idiomatic one that a native speaker of English might actually use. There is some room for improvement here.
Not really. A "sequel" is a narrative work--fiction, drama, film, history, journalism-- that deliberately continues or adds to the story in a previous work. A follow-up can be different things, but is generally used to mean some kind of further investigation or treatment of an issue. You ask a "follow-up" question if the answer to the first needs amplification. You go back to a doctor for a "follow-up" visit after an initial consultation.
In my ancient times, regarding Saxon genitive, school teacher marked wrong the "'s" after nouns: only for names allowed! I argue the rules has changed, as Duolingo marked wrong "book continuation" correcting into "book's continuation" and nobody yet commented on this matter here.
"What is the continuation of the book" is marked correct but "what is the follow up to the book is marked wrong". This is incorrect because the book is a finished item. A continuation to a book would have to be an epilogue or a revised and updated version of the book, maybe adding new chapters as is often the case with biographies. A follow up to a book might be another book, a sequel or maybe even a film of the book. Continuation just does not fit in native English.