Why did Giovanni Fontana write his book in cipher?

[Mark Knowles]

It's a simple substitution cipher. A mnemonic one. It takes 10 minutes to break it.

I don't think it was created for secrecy. It was used in manuscripts that showcase his engineering ideas. I think the cipher is a way to demonstrate his abilities when combined with the engineering diagrams.

Is it your contention that simple substitution ciphers were never used for secrecy? Looking at 14th century diplomatic ciphers it appears that they used simple substitution ciphers for secrecy then.