Write a letter expressing your inability to execute an order.

Dear Sir,

This is to express our gratitude to you for placing an order of 6^th instant. However, we regret to inform you that we are unable to execute your order since the books in question are out of stock at present. We expect to receive our consignment by the end of this month.

We shall feel obliged if you allow us to send the books by the first week of next month. We will appreciate it very much. 

Thanking you   

Yours faithfully