If I had to guess, I'd guess Henry Dreyfuss's Symbol Sourcebook. It was published in 1972, and it seems plausibly the sort of book someone like Susan Kate might have had to hand in the early '80s. https://www.societyofsigns.com/projects/symbol-sourcebook
Symbol Sourcebook would’ve been my first guess, too, but I just glanced through my copy (7th printing, 1977) and didn’t see the ⌘ symbol. The closest thing in the Graphic Form Section is a symbol for “Atomic d orbital,” but it’s clearly not the same one that inspired Susan Kare.
Interesting. The left side of the slide at 15:43 in the video is definitely from page 27 of Symbol Sourcebook, but the detail of the ⌘ symbol doesn’t seem to be: not only could I not find the symbol, but also its caption (“FEATURE”) is set in Helvetica rather than Univers as used in the book.
I have a suspicion that she may no longer possess or even remember the book in question. Heaven knows I wouldn’t were I her, but my memory is atrocious.
If I had to guess, I'd guess Henry Dreyfuss's Symbol Sourcebook. It was published in 1972, and it seems plausibly the sort of book someone like Susan Kate might have had to hand in the early '80s. https://www.societyofsigns.com/projects/symbol-sourcebook