I landed a few hours ago from my trip to Istanbul, where this book hasn't left my bag except to be eagerly read. This is not a classical, full of pictures, timetables, tips and tricks, and restaurant list guide. It's a guide that provides itineraries through Istanbul, describing in full historical and artistical detail what you find along the way. Includes more information and more places than any other guide, and it's very useful for reading while visiting; stop for a while, find the place in the index in the back, and read aloud for the benefit of your group, and to illustrate everybody. You'll be fascinated with the history, art and technique of Sinan, know about the descriptions of Evliya Çelebi and be able to spot Iznik tiles on sight. Granted. It's even more useful if you read, before going, Istanbul: The Imperial City
by one of the authors, but, in any case, it stands by itself.
The only problem I find with this is that you'll have to complement it with, at least, a map (which you can ask in any of the Tourist Information booths, or at the hotel) and maybe some information on opening and closing times; some kind of prioritization would have also been a good idea, but, in any case, it's an excellent book to carry when you're in Istanbul, probably the best.