Ptolus is a deluxe campaign setting describing a single city. But what a city! There are virtually infinite dungeons beneath the streets, the greatest evils in the worlds reside within the walls of the city, as well as the greatest good.
It is amazingly detailed, but not dull. I challenged my players to come out with a practical question I couldn't give an answer to from the book (e.g. "where can we get a massage?"). They couldn't. It is edited wonderfully with references, giving you an easier access to the material in the book.
It is very easy to create adventures just from the stuff that's inside the book, but a few great adventures are available: The banewarrens, Night of Disoolution and Queen of Darekness.
If you just get one more 3.x product ever, my suggestion is: get Ptolus.