There are a number and the three mentioned above are very good and among the most recent. The Color Encyclopedia of Hostas is a nice reference that sorts by the colour. But even the best of these are now getting fairly old and with hundreds of new hostas being registered every year, they get our of date very quickly. Stay away from the Paul Aden Hosta Book because Paul has been shown to have fraudulently registered many hostas under his name that were, in fact, stolen from others. So while there's some good basic info in it, there's also a lot that is no longer valid.
