Kasselmann's book is undoubtedly great for a rich database of plants and wealth of ecological data. Her planting philosophy, though, is disputable. But still a must-buy.
I would recommend also the Baensch Encyclopaedia Vols 1-3, as their plants and algae section is highly comprehensive, with extensive commentaries on individual varieties. Vol. 1-2 can be found on occasion at Kino or Borders, Vol 3 focuses on rarer species, and is likewise very seldom seen.
Barry James' Aquarium PLants is a good no-frills newbie book. So is the Scheurmann.
The proprietary books understandbly lean more on the technology than the science. But the big Dupla book (available at Aquatechnic) is quite thorough, and so is the existing Dennerle. Tropica's is for their own plants, while the newer Oriental book is a better guide to what's available locally. The other end of the scale is Diane Walstad's near treatise (only via mail/online order)
IF you can read Taiwanese Chinese, the white hardcover book called something like "native plants of Taiwan" semms like a good reference as well.
Amano stuff is eye-candy, that's about it.
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