How to update record using Entity Framework 6?

I am trying to update a record using EF6. First finding the record, if it exists, update.
Here is my code:

var book = new Model.Book
{
    BookNumber =  _book.BookNumber,
    BookName = _book.BookName,
    BookTitle = _book.BookTitle,
};
using (var db = new MyContextDB())
{
    var result = db.Books.SingleOrDefault(b => b.BookNumber == bookNumber);
    if (result != null)
    {
        try
        {
            db.Books.Attach(book);
            db.Entry(book).State = EntityState.Modified;
            db.SaveChanges();
        }
        catch (Exception ex)
        {
            throw;
        }
    }
}

Every time I try to update the record using the above code, I am getting this error:

{System.Data.Entity.Infrastructure.DbUpdateConcurrencyException: Store
update, insert, or delete statement affected an unexpected number of
rows (0). Entities may have been modified or deleted since entities
were loaded. Refresh ObjectStateManager entries