The argument that you would only use a system like this if you don't want people to know there's a code bears a gaping defect in logic: If you didn't want people to know there was a code you would use a cardan grille, therefore if you use a cardan grille it is because you don't want people to know there is a code. Minus 5 internet IQ test points for anyone who tripped up on that one.

Steganographic techniques can be used to add another level of security to an existing system. The grille need not be the medium, the user can be directed to the appropriate pages and letters by whatever means are available (see also: numbers stations)

On the risk of discovery, if someone is going through your books you've already aroused suspicion, you don't need to worry too much about throwing up flags. what you can do at that point is make it as difficult as possible for them to intercept and decode your communications. so you have a reference book, disguised as an exotic scientific work, of which you must both have physical posession and understanding in order to translate the messages of which you must also have physical posession. given the year in which this book appears to have been written and the lack of education in such times, the book would appear to be nothing out of the ordinary to virtually everyone. the wheeze is even more effective if the user can fairly claim to have kooky scientific texts for professional purposes, in which case a strong defence is availible even if challenged.

if this is the case, the probability of translation is of course zero :o)