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.
