A suspicious parcel...

It's nearly Christmas time,and I have asked for 2 codex's,the tau codex and the marine codex.A few days ago,a parcel arrived in a ups van.When I got my first 40k set,It came in a ups van.While my mum was out of the study,I grabbed my guard codex and compared in size to the parcel, which by now I had established was a book.It looked the same size as my guard codex.So I think mum has ordered a codex.Trouble is,which one is it...