public object Add(object item)
        {
            BookVariableInfo bookVariableInfo = (BookVariableInfo)item;

            using (BookMarketDbContext db = new BookMarketDbContext())
            {
                db.AddBookVariableInfo(bookVariableInfo.Id,
                    bookVariableInfo.ProductCount, bookVariableInfo.Price);
            }
            return GetAll().bookVariableInfoes.Last();
        }