I’ve always been a fan of the Admin Bookmarks plugin and suggested a friend that he turn it into something that all users could use (which became the User Bookmark app: https://moodle.org/plugins/view/block_user_bookmarks).
Hrvoje Golcic recently contributed a new addon that extends the functionality to allow bookmarking within even Book module chapters. According to the description,
Users can create their own bookmarks at any desired position within the text Moodle Book chapters and they can later access it quickly using “jump focus” switch. All the bookmarks will be listed within the block once the user open desired chapter again. This might be useful to anyone who likes to have side notes next to their learning material. The tool can increase accessibility for blind and visually impaired people. Each user can easily create, delete and rename their own bookmarks.
If you’re looking for ways to allow students to save their place and get back this might be a good plugin to watch. Please note that this plugin is currently in the category called “Incubating” which is for plugins which might bend/break some of the Moodle plugin development rules but show promise.
Start monitoring it, become the maintainer or download it for testing at https://moodle.org/plugins/view/block_bookmarks