I want to purchase Greek books as in books written in the Greek language how do I do this ?

Canada does two languages: English and French. If you want books written in Greek, then try a bookstore in Greece. Of course you can search Canadian book stores for books in Greek.

When you can't find what you want in the iBookstore for your country, the best fix is usually to look in one of the other stores which also provide free ereader apps, such as Kindle, Nook, Kobo, Googlebooks, Sony.