Failure to deliver on time and lying

Ordered two books on January 5th for my child and they promised to deliver within 3 days after their accounts department got the money. It took them two weeks to deliver only one book. Now we at the end of January and the second book isn't delivered yet. When I enquire about delivery of the second book they say they will investigate and get back to me but nothing comes of it. My child is falling behind as a result.