Instead of saying:
- "It's only you who have that book, isn't it?"
can we say,
- "Isn't it only you who have that book?"
Instead of saying:
- "It's only you who have that book, isn't it?"
can we say,
- "Isn't it only you who have that book?"
You could. However, both of those variants sound rather stiff and old-fashioned to my ear. Today, a more conversational wording would be "It's only you that has that book, isn't it?"