Xelement remove problem

I have an xml file something like the one below. I'm just trying to remove one element, but I think I am missing out on something. I am getting this error below .But There is something strange .After close solution and reopen than build project visual studio says succeed but i cant do it more than one time if i try to build second time then i get this message again .I have tried five times this .And every time same result .Is it normal?  System.Xml.Linq.XElement' does not contain a definition for 'First  and no extension method 'First' accepting a first                     XElement document=XElement.Load(Server.MapPath("data.xml"));     var x = from veri in document.Elements("Book")     where veri.Attribute("ID").Value == "5" se ...