[wsh]

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.

[macintux]

Around 15:30 in this video she talks about it, and there’s a slide showing other symbols that may or may not be from the same book.

https://vimeo.com/151277875

[wsh]

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.