I thought it self-evident that they would fly planes into the World Trade Center. Mainly because (a) they said they wanted to destroy the World Trade Center; and (b) they said they wanted to fly planes into buildings.
1+1=2. It wasn't complicated.
I've read Tom Clancy books too, but you are a long way from proving that we had real intelligence that they were implementing this kind of plan.
We had precisely that. Here's a public example that I was aware of: Operation Bojinka.
Are you saying it was self-evident that they would strike on that day?
No, not at all. I was however suspecting that there might be a strike on September 18, 2001 (Rosh HaShanah) because it was rather clear from the State Department notices throughout that summer that there was an extremely high threat of terrorist attack and the Jewish holiday seemed like as good a conjecture as any.