This encyphered engineering book is interesting, maybe?

[Mark Knowles]

Knowledge that they wanted to keep secret. They were concerned to keep their intellectual property from being stolen by others. There were no protections like patents.

Ok but the images are right there, unencrypted. Everybody can see how to build that trebuchet looking thing just by looking at it. Professional engineers don't need IKEA isntructions for building trebuchet. So there is no point to encrypt the text "Step 1: take a screwdriver, Step 2: take 3 screws of type A". There is simply no point to encrypt these things if you can already see the diagram.

Well, as I say, there was certainly a fashion for writing in cipher at that time. Maybe, it made someone's manuscript seem more credible or respectable to others. Did Giovanni Fontana need to write his manuscripts in cipher?

But certainly, intellectual property reasons were common.