Why did Giovanni Fontana write his book in cipher?

[Mark Knowles]

So are you arguing that if the Voynich manuscript is in cipher the motivations of the author were the same as Fontana's?

No, of course not. The Voynich has no plain text. There is no comparison between the two works. Fontana's was a highly technical work which aimed to delve into the workings of his machines.
Fontana's cipher is not difficult to decipher and follows common techniques of his time.

Some might argue that the Voynich manuscript was a highly technical work.

Fontana's cipher seems a bit primitive for the time. However I get the impression that Fontana didn't realise how primitive it was. I would suspect that was because he had little or no background in ciphers. I would think the author of the Voynich had a very strong background in the latest cipher techniques, which would account for the much greater level of difficulty we find in deciphering the Voynich.

If you argue that the Voynich is not consistent with ciphers of the time, I would say watch that space. There was more inventiveness and variation than you might think.

To me Fontana's works seem to be the closest parallel to the Voynich.