Thanks Cristian, there’s two different problems.
One is about my custom taxonomy. That’s the /library/ example I describe above. I found it was rather easy (once you find the course code) to define the custom taxonomy to my custom post type. The code allows me to define the slug. The thing I was missing is that the plugin doesn’t support the slug option, and the taxonomy pages would appear at the root. I want them to appear within the same slug as the custom posts (which just seems logical to me when custom taxonomy is attached to a post).
Anyway…I got all my custom post and custom taxonomies working. That was the /library/ problem.
The other problem is the /blog/ (which you helped me with in a different thread. I needed to make the change to the WP Settings permalinks to support my /library/ custom post. Your suggestion got that working just as I needed, but had a side effect on the /blog/.
At first I thought it was working. All my blog posts are in fact within the /blog/ slug as I want. But what I noticed last week is the blog’s main page is no longer the page I created. That slug has been replaced by an archive page.
My blog’s home page is an actual WP ‘page’ (not a ‘post’) that I designed. It used a shortcode to show the list of blog posts. Now after the changes the /blog/ location is no longer showing the WP page I had created. Looks like WP is now using that slug for the archive permalink instead of the URL to the page I had created.
I suspect this means I’ll have to create a custom archive template in PHP for my blog posts. That’s unfortunate since it’s so much easier to create pages with the WP UI and my theme, rather than writing PHP code to layout a page.