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Dick King Smith's book "The Water Horse" is about Nessie. They made a movie of it a year or so ago. It's probably about 4th grade reading level.
Also, this is an adult book but would be good for a high reader:http://search.barnesandnoble.com/Field-Guide-to-Lake-Monsters-Sea-Serpents-and-Other-Mystery-Denizens-of-the-Deep/Loren-Coleman/e/9781585422524/?itm=6
Hope this helps: If you have any other book related questions I am a mom and children's bookseller for over 13 years and I just started a blog reviewing kid's books. I love helping kid's find just the right book that will spark a love of literature!
Tanya @ www.books4yourkids.com