The patch and tweak folks have a bunch of symbols here: http://patchandtweak.com/symbols/. They’re designed to describe patches, not modules, but may provide some inspiration.
In the book, they mention the following prior works:
- Liner notes for Nonesuch Guide to Electronic Music (Beaver and Krause, 1968)
- Electronic Music: Systems, Techniques, and Controls (Strange, 1972)
- Patch Notation: A Business Approach (Bugg, in Synapse March/April 1978 (PDF))
- Serge Modular Music System manuals.