Ability to apply and include CDN resources into your superbook is one of the best features of Bubblin. It's an outcome of bringing up web & books together.
There are two ways to apply CDN resources into the HEAD of your page:
Templates are an ideal way to configure the HEAD of your entire book. For example, if you're writing a book on mathematics or on elementary particle physics then it is likely that you're going to need Mathjax (Latex / Katex) on all of your pages.
Use templates for that.
If only a specific page of your book requires an external resource then use HEAD feature on the editor toolbar (near 'Prevew' button) as shown in the screenshot overleaf.