All of Rick Steves' books are invaluable for the traveler who has a medium size budget. Rick knows all of the inexpensive places to see, eat at, and lodge without breaking your wallet. He rates attractions on the basis of interest and importance and recommends how much time to spend there as well.
His philosophy is to stay away from the tourist restaurants and hotels and instead live in a neighborhood, frequent the restaurants where the locals go, and interact with the people for the best time.
We have used many of his books and found them to be most helpful. I recommend them!