> Is there any evidence this book ever saw widespread use
Commercial code books like this were quite common and available in office supply shops apparently into the 1960s at least. The point of these code systems was not to keep a secret but to significantly reduce the cost of sending telegrams which charged by the word (and were handled by humans so you couldn’t save money by omittingspacesbetweenwords — the clerk taking your order would just put a stroke between the words and would declare the charge.
I originally learned this because, back when we had paper appointment books, they also often included appendices with handy info: time zones, common abbreviations, etc and when I was a kid, vestigial telegraph codes. Dictionaries often had sections with stuff like that to, and these were quite important in translation dictionaries.
Into the 70s phone trunks were quite limited and you often had to reserve a long distance phone line ahead of time, so telex/telegraph survived quite late. I was sending email and casually logging into remote machines over the net in the late 1970s yet to book a plane ticket in the 1980s could still require sending a telex.