When one should chooser "чьи это книги?" and when "чьи эти книги?". Or are these forms completely interchangeable?
Will it make sense to you if I say, "because это is a picture-frame rather than a tag"? In other words, это refers to the abstract, uncountable "stuff we're looking at", rather than to be a marker attached to the books themselves, as in the English "Whose books are these?". You can ask Чьи эти книги?, but it will sound weirdly "off", at least if you were pointing to the actual books. The variant with эти can (at least in some cases) sound natural when what you're asking is "whose are these books?" rather than "whose books are these?" – i.e. when "these" is a contextual reference rather than a literal indication. But even there, I must say, I can't quite come up with satisfactory everyday examples. The abstract, independent, generalising это is just so much more common.
In such cases people say "it's just as it is". Note:we use a declined form of this determiner«Эта книга чья? Эти книги чьи?» only before the noun in emphatic sentences.Used in post- position the pronoun is not declined and the sentence sounds more neutral. Though we can't deny grammaticality of a declined pronoun used in postposition, we don't use it as a rule.