Hi Tim,
I am not sure if I can tell without actually seeing the issue. I suspect that there might be a syntactical error, maybe the documentation is not typed in correctly, or it uses a different value, or maybe there is a typo in the custom field name.
If the filter works with ‘books’, then there is probably only a typo in the word ‘documentation’, or something similar. If you want, you can include a URL and back-end details, and I will check it out.