"His second book."
Translation:Buku kedua dia.
Is "Buku keduanya" also correct or not? Because "keduanya" also means "both" I think.
It's correct, but you're right that it can be ambiguous. So you'll have to consider the context. If it's clear from the context that the phrase means "his second book", then by all means, use it. If it isn't clear, then you can use "buku kedua dia" (or buku kedua miliknya) to make the meaning clear.