I am on the official Parc Guell website and wonder if I should book a Parc Guell tour or just the park ticket?
Depends on if you want a tour or explore on your own. Only you know the answer.
I don't think a tour is necessary. We just booked tickets and used the Rick Steves guidebook a bit.
As I recall the tour was not that much more money. It was brief, helped us see some highlights, get our bearing. Then we went back and revisited the places which interested us.
Actually, I don't think a guided tour gives you any special access; to my knowledge these days with the regular ticket, you can visit the same spaces you'd visit on a guided tour.