Price and identification guides can explore the history of a company, show marks and other company resources usually much better than a single web site or two. Although it's difficult for a printed book to have current prices in a time when prices can fluctuate so much, you can still find out the relative values to pieces in the same line. Here are a few picks from my collecting library.
Collector's Encyclopedia of Metlox Potteries: Identification and Values
Although there are plenty of good pictures and catalog sheets, I feel this book's strength lies in the extensive history of the company, as well as complete descriptions of the various lines. Gibbs helps explain away the confusion of the different divisions, numbering system and different markings.
Warman's McCoy Pottery
If you're a beginning collector, this McCoy book is perfect for you. I like the way author, Mark F. Moran, groups like-items together for comparision, especially in the cookie jar chapter. For instance the two different McCoy Christmas trees, one silver and one gold. Or the two rocking horses, one cold-painted and one glazed. Another feature included is a few "fake" items next to the real McCoy. The book makes extensive use of old McCoy catalog sheets -- always a great reference.Hull Pottery Decades of Design
Lots of pictures, especially good images of the marks used on the pottery's wares. Decades of Design is just that -- the book is laid out by the timeline items were produced, which I found little harder to use without lots of turning back and forth to the index.