Pay no mind to me. My wife and I think the DK.com travel guide books are the best to investigate the interesting things of a city. Why? Pictures, drawings, mini-maps, enough text to inform, but not enough to read like a history book. When it comes to food recipes and travel books, my wife and I strongly prefer a few pics. So, I'm just putting links to guides here that I plan on buying or ones that we already have so that when I do additional school visiting and go to Business School, I'll remember that I bookmarked these books here.
It's a shame that there isn't one for Philadelphia.
You may be familiar with AAA, Lonely Planet or other. I would suggest you check these DK.com ones out.