On a trip to Istanbul last fall, we followed a Perrin's People recommendation and used Karen Fedorko Sefer at Sea Song Tours. It would not have been the same trip had we not used this excellent service. Nigar, our guide, was experienced, knowledgeable and warmly personal. We saw parts of Istanbul not available to the casual traveler. Our only mistake was not using Sea Song for Ephesus. We took the ship's tour, and it was pitiful and much more expensive. We highly recommend Sea Song Tours.
Sea Song Tours, Istanbul; 011-90-212-292-8555, http://www.seasong.com