Url path is going wrong when I rewrite it using HttpModules

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C#

Hi,
I am using visual studio 2010 for a urlrewriting with httpmodule.
The code which I am using just like below

HttpApplication MyApp =(HttpApplication) sender;
            string MyOldPath = MyApp.Request.Path;
            string OriginalURL = string.Empty;
            switch (MyOldPath.ToLower())
            {
                case "/home":
                    OriginalURL = "Modules/Users/Home.aspx";
                     break;

                case "/library/getbook":
                     OriginalURL = "Modules/Books/BookShelf.aspx";
                     break;

                default:
                     break;
            }
            if (OriginalURL!=string.Empty)
                MyApp.Context.RewritePath(OriginalURL);
            string strURL= MyApp.Request.Url.ToString();

First case is working. But in the case of second case the url is rewritten by an additional "Library" root with the existing one. That is when hit the case "/library/getbook" the url will be Library/Modules/Books/BookShelf.aspx. Could you please help me to rewrite the root too.