Aquarium Plants Manual by Ines Scheurmann is probably the best "beginners" book on aquarium plants. Its cheap, easily available and very well written.trenac said:Two good books for the beginner are...
Plants for your Aquarium by Wolfgang Gula (more general worded/details )
Aquarium Plants Manual by Ines Scheurmann (more scientific worded/details)
These where my first books that I still reference on occasions.