"Are these their books?"

Translation: Sind das ihre Bücher?

[2xFlowerIron]

Why can't I use "dieser/diese" instead of "das"?

[mizinamo]

diese would be a translation for "these ones" -- i.e. if you had previously been speaking about books and were now pointing to a specific subset of those books. So it would be short for diese Bücher "these books".

If you're introducing these books newly to the conversation by pointing at them, you would use neuter singular (dies or das), regardless of the gender and number of the thing(s) you are introducing.

[Mike306897]

Couldn't "Sind das ihre bücher ?" also be translated into "Are these her books ?" ? If not, why ?

[mizinamo]

Yes, it could.