For aquascaping ideas and sheer inspiration you can't beat Takashi Amano's Nature Aquarium World series (books 1 to 3). Around £20 each.
Jeff and Mike Senske's "The Inspired Aquarium" is a good book but doesn't go into plants in detail. There's some stunning tanks in there though.
For practical advice I consider most books outdated, and some are plain wrong in there advice. Peter Hiscock's Encyclopedia of Aquarium Plants is ok for beginners.
The best book on actual plants and their care is probably Christel Kasselmann's "Aquarium Plants". She also has a smaller book called "Planted Aquariums" that goes through setting up and maintaining. It's ok but very bias toward Dutch style aquascaping and methodolgy and some advice is wrong IME. The Tropica handbook is good too but you can get most info, and more from their site - www.tropica.com.
For non-CO2, soil-based methods (low-tech/natural) then Diana Walstad's Ecology of the Planted Aquarium is very good. It's a very scientific read.
For high-tech set ups the Dennerle guide is ok but its more a marketing tool for their products. It does have a good section on the plants themselves though and there's some half decent photos too.
I consider the Internet my favourite reference these days. This forum is good for advice with a few highly experienced members active including Tom Barr, one of the world's top experts in this field.
There are other forums that specialize in planted tanks too. The Barr Report, The Planted Tank, Aquatic Plant Central and Aqua Essentials forum are all excellent.
