1. https://codex.wordpress.org/Styling_Page-Links
This page for styling page-links is informative, you get an idea of how you’d change other similar features from the examples. More generally, the entire WordPress reference/documentation offered here is very useful.
2. Captcha plug-ins
The captcha plug-ins referenced in the book appear to be deprecated, but I think it could be something worthwhile to implement. I found one that’s newer and has many features: https://wordpress.org/plugins/google-captcha/
3. https://wordpress.org/plugins/wp-optimize/
This plug-in is used for cleaning and optimizing. The default options for having an overview of unneeded backups, drafts, and other clutter aren’t very robust, so something like this could be very useful to keep things organized.