Soor Al-Azbakia is a large collection of bookstores in Al-Attaba. It’s a place for all kinds of books where, with some digging, you can find some really valuable books at good prices (price negotiation is often expected).
Although this is the best place in Cairo to buy and sell used books, you can also find some new books and best-sellers. You can even find out-of-print books. You’ll be shocked at what you can find there. Of course, the majority of the books will be in English and Arabic, but you can also find other languages like Spanish, Chinese, Korean, Russian and much more. I’ve personally bought some French novels at a great price (10LE each) which is much cheaper than any bookstore price.
You can also find university books and books for children and basically almost everything, but you’ll have to be willing to look around enough to find the hidden treasures.