Sorry, these books are on reserve.

对不起，这些书不外借。

I'm sorry. I'm on reserve.

对不起，我已经预订好了。

I'm sorry. I'm on reserve.

对不起，我曾经预订好了。

That book is on reserve for a professor.

那本书被一位教授预定了。

That book is on reserve? for a professor.

那本书被一位传授预定了。