I agree with Mmario, anything by Diana Wynne Jones if they like Harry Potter. DWJ books were my absolute favourite when I was wee, and many of them have a similar 'magic hiding just below everyday mundanity' theme to HP stories. For reading aloud maybe "The Ogre Downstairs" or "Charmed Life", for older kids to read to themselves then "Power of Three" or "Fire and Hemlock".