01.AgeRestriction

 public static string GetBooksByAgeRestriction(BookShopContext db, string command)
        {
            var titles = db.Books
                .OrderBy(b => b.Title)
                .Where(b => b.AgeRestriction
                             .ToString()
                             .ToLower()
                             .Equals(command, StringComparison.InvariantCultureIgnoreCase))
                .Select(b => b.Title)
                //.OrderBy(t => t)
                .ToList();
            var result = String.Join(Environment.NewLine, titles);
            return result;
        }