If you're in the market for a piano, in most cases we recommend you read Piano Buyer first. It's an easier read and may satisfy your needs well enough. It's also more up-to-date than The Piano Book, particularly for new pianos, and is the only publication of the two to cover digital pianos.
The Piano Book covers acoustic pianos in greater depth and a little more technically than Piano Buyer; however, it's still very accessible to the layperson. The Piano Book also has more specialized information than Piano Buyer about used and restored pianos, piano moving, and piano servicing. We recommend it as an additional resource. For information on current brands, models, and prices, The Piano Book is completely out of date -- see Piano Buyer instead.