As Bates (2014) noted " an editor or a co-author cannot mark up drafts in Pressbooks"
TinyMCE Annotate (https://wordpress.org/plugins/tinymce-annotate/) provides that function. However, it is not compatible with Pressbooks. In addition, testing revealed that markup is displayed to anonymous users as I have found no granular control to hide markup.
A granular accounting visibility control and a clear all markup (in addition to the individual default) could be useful.
Although Hypothesis is great for those how want to login again to another site, for open e-textbook sites having internal authoring markup functional is useful for developing content.
Bates, T. (2014, September 26). Writing an open textbook: a mid-term report on the technology [blog]. Retrieved November 6, 2017, from https://www.tonybates.ca/2014/09/26/writing-an-open-textbook-a-mid-term-report-on-the-technology/