I doubt very much there will be anywhere selling English books in Kartal. Some of the bookshops in Kadikoy have a limited selection of english books but your best bet by far is to get over to Taksim to pandora kitapevi on buyukparmakkapı sokak. The original shop is on the left hand side of the street and is now the Turkish dept..they have recently opened a shop directly opposite this which is the English and foreign language bookshop. All books seem to be at the recommended publishers price. ie no mark ups on the price printed on the back of the book. There are one or two other good bookshops on the European side but Pandora's is for me by far the best place in Istanbul and indeed in this country for English books.