I have all three of the Natural Aquarium Books, and I agree with the others that they are primarily coffee tables books. They make great eye candy and can give you inspiration on things to try, but will give you little in the way of an actual "How To". It's also good to note that whether due to translation issues or somesuch, many of the plants in these books are incorrectly identified. That being said, if you have some money to burn they are definate worth buying. The first book has a wide range of tank sizes, the second focuses primarily on small tanks, and the third focuses on large tanks. The first two are by far my favorites since they have scapes that are a scale closer to what I'm actually dealing with.
Lately I've actually been collecting some older books. While some of the information is out of date and/or they are highly focused on selling a particular manufacturer's product, it's still quite fun to browse through books that were "The" books to own not too long ago. So far I've collected the following:
"The Optimum Aquarium" by Kaspar Horst and Horst Kipper (Heavy Dupla Advertisements)
"Encyclopedia of Water Plants" by Dr. Jiri Stodola
"Aquarium Plants: Their Identification, Cultivation, and Ecology" by Dr. Karel Rataj and Thamas Horeman
A more recent text that I would recommend is:
"Encyclopedia of Aquarium Plants" by Peter Hiscock
Books currently on my wish list include:
"Aquarium Plants" by Christel Kasselmann
"Planted Aquariums: Care and Maintenance" by Christel Kasselmann
"Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise for the Home Aquarist" by Diana Walstad
"Aquarium Plants Catalogue" by Tropica Aquarium Plants