Good news from the https://h5p.org/january-2023-release-note: The H5P Interactive Book activity now features the Save Content option.
Interactive Books (IB) can contain a fairly large number of pages and diverse types of sub-contents. It can be frustrating for a student browsing an IB to find that, having "done" a certain number of pages & activities, if they leave before the end, when they come back to the IB they have to start from zero.☹️
With the latest version of IB, if the Save Content option is enabled on your site, upon re-entering an IB, the student finds themselves on the page they had previously reached.
Please note that in Moodle this only applies to the contributed hvp plugin (the black H5P activity icon). One more reason for wanting that Save Content option to be added to the Moodle h5p activity (the blue icon), with a long-awaited fix to MDL-67789 with its 70+ votes.