If you are a book freak and love to read foreign books then your direct address should be Pandora Bookstore. They are a totally separate shop dedicated to (mostly) English books varying from literature to hobby, educational to history. But I have to say that they have the richest variety of literature novels in İstanbul.
If you are looking for books to buy about Turkey, translated Turkish authors etc Pandora is the best spot for you. Obviously there are a couple more book stores around but Pandora has the best atmosphere, location and the best possible friendly personnel.
As a local I always go there to feed my book thirst and there I noticed a lot of expats with normal tourists.
They have moved the English bookstore to their new two storey building which is just fabulous. I can easily say that it’s the best English title bookstore in Istanbul.