On visiting Istanbul last November, I took the DK book as I've had great experiences with others in the series - this one lived up to my expectations!
We stayed primarily within the historic neighbourhood of Eminonu (where the Blue Mosque, Haghia Sofia and Topkapi Palace are). The streets are labyrinthine but the maps are accurate so we had no trouble finding our way around. At every location, the book gives excellent descriptions and the cut-away illustrations just add to the detail. In particular, we found the suggested walking routes very useful.
I highly recommend this book, but it wold be a good idea to read most of it before you go so that you can really look at the place as well as reading about it. The city is fantastic - I'm obsessed with the place now - nad intend to go back several times, armed with this book.
Some tips -
Our first problem was getting from the airport, as everything seems very vague. We ended up getting the bus into town and then a taxi to our hotel (he got lost!), but on the way back we took the airport shuttle from our hotel. In a city like this, USE THE AIRPORT SHUTTLE BOTH WAYS! Expedia give you the option, I'll definitely tick that box next time.
Take plenty of money - food is quite cheap but entries into places soon mount up. They aren't expenseive - 10 lira is about £4.50 ish, but you might pay four or five fares in a day, which quickly eats at your funds.
If you're shopping, only accept tea where you actually want to buy something (they're offended if you refuse tea, but even more offended if you drink it without buying anything), and HAGGLE all you can :-)
Have a great time!