I ordered one (for $15) as I had read on the boards that you can get good discounts at the Universal hotels. However, the reduced rates are not available over Easter (when we are there).
Holly is probably right - the coupons are just like the ones you get in the free booklets when you are over there.
My suggestion is that you check out the rates on the Loews site for the dates you want and put ENT as the promo code. If the price is cheaper with the ENT code, then make the booking and order your Entertainment book. You don't need the card (which comes with the book) till you check in.
It may be easier, though, to just ring up the hotels and check whether ENT rates are available.
Julie