Turkish Bookshops + Books About Turkey
For travel planning and information, we used the Lonely Planet Turkey guide, which was pretty good. I also had a Globetrotter Istanbul guide, which was useful for working out which sights to see but had little practical information. There's no shortage of travel books about Turkey, though!
As background before the trip I had read Chris Morris' The New Turkey and started on Zurcher's Turkey: A Modern History. Earlier reading that proved useful included Between Two Worlds: The Construction of the Ottoman State and Robert Irwin's Islamic Art.
I also got a bit of advice about Turkey from friends before leaving.
There's a nice bookshop Galeri Kayseri in Sultanahmet, with a huge range of books about Turkey. The books here are all full price, however.
In Beyoglu, on and near Istikal Caddesi, there are lots of bookshops, both new and secondhand, that have good English-language sections.