My understanding of plaguerism is that it pertains to copying someone else's written word almost verbatim.
There are many cases of this where you can hold up two books and see entire sections and paragraphs copied verbatim with maybe a change of location, name, and an adverb change here or there. But the two sections will be 95% identical.
Now, if two people write different stories about Count Dracula. Big deal, that isn't plaguerism.
Now, in terms of stealing someone else's concept and doing a re-write. That is more of a gray area that has some subjectivitity.
I know that Dan Brown was actually sued along these lines with The Davinci Code. Because the ideas behind the Davinci Code are nothing new. He was just the first to wrap them all together. There was another author who did something similar a few years prior to Dan Brown, but even in that case, the court ruled Dan Brown was o.k.
http://www.americasdebate.com/forums/index.php?showtopic=12286
In a nutshell, as long as you write the book without looking at another book while you write, you are going to be o.k.
Mel...