How Do I "get from model repository by id"

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MVC4

Please I have a forum app and i have an e-Library, and i want to "get from model repository by id in my controller. How do i get that done. See what i have:

public ActionResult _BookDetails(string id)
{
var mybook = db.Library.Where(a=>a.LibraryId = id.ToUpper());
//if (book != null)
//{
// ViewBag.p = "True";
//}

ViewBag.mybook = mybook;
return PartialView();
}