There's no one book - it's a visually oriented mindset. But if I were to suggest a few books that are web specific:
'The Principles of Beautiful Web Design' by Jason Beaird,
'transcending CSS' by Andy Clarke
'Web Designer's idea book' by Patrick McNeil
Otherwise, check out the graphic design section at Amazon (or your nearest big bookstore) and look online at what you consider good designs, and try to analyze what is is they're doing right. Well crafted graphics, balance, color choices...?