The Gateway to the Moon is not a guide to the NASA Kennedy Space Center as such but describes the early history of America's manned space programme.
There are no guide books that are written solely about the NASA Kennedy Space Center but there are several books on Florida that have chapters dedicated to the NASA Kennedy Space Center.
Therefore you should probably choose a book based upon what else you plan to do in Florida.
There are also several good books written about the early days of the NASA Kennedy Space Center when mankind was aiming for the moon.
Note that most guide books are re-published annually so you can often pick up a bargain if you don't mind last year's edition. You won't get the very latest information but you could save yourself quite a bit of money.
Also look out for special deals on most Internet book sites where you can often buy two related books at a special discounted price.