I've enjoyed having the time to take tours from English tour guides in France. A good tour guide adds a lot of depth and detail to a place without overwhelming you by boring or irrelevant detail. There are a lot of experts in France if you have the time and patience to find them. I already wrote about my trip to Chartres to have a tour of the cathedral by the amazing Malcolm Miller. There's good options in Paris, too.
In the cathedral vein, I just finished a very good tour of Sainte Chapelle as guided by Vicki-Marie Petrick. Ms. Petrick is a PhD student of middle ages art with a bit of thespian flair. She did an excellent job of explaining the role of the church and its relics in bolstering Louis IX's claim to the throne. She also gave a good explanation of the stained glass, in particular the Book of Judith window with its proto-feminist story. Her expertise was generously shared, I definitely suggest you take her tour if you are in Paris.
A good source for walking tours of Paris is Paris Walks. It's a fairly casual outfit, interesting and entertaining and with a wide menu of possible tours. I took two, one in the Marais and one in the Left Bank. The guide deftly wove together history specific to the streets we were on with a general view of how the neighbourhood fits into modern Paris. Entertaining, a bit bookish, and a very satisfying 90 minutes. If you're planning on visiting Paris take a look at their schedule and pick a couple that sound interesting. Then just show up and go for a walk, couldn't be easier.