There is a big problem with art books: it is hard to browse vast quantities of images quickly, slice and dice art works by artist, time, genre. A good painting requires time to appreciate, once you found it. However, the finding is the difficult bit.
The iPad is a wonderful device to navigate huge image data bases (earlier post). I am a bit late to discover Art Authority for iPad, an application that make this a reality for art. Over 1,000 (Western) artists, with each painting properly documented plus links to Wikipedia for more information.
Most art books show the same "greatest hits" paintings, not spending paper on less well-known works by artists, paper publications cut off the long tail. Not with Art Authority that shows works beyond the beaten path.
$10 well-spent.