Sorry for the late reply. Not sure if this would work for you, but create the H5P activity in the same course as the book. Then when you embed the H5P activity at the last chapter of the book, when students completed the embedded H5P their score will be recorded in that H5P activity. To make the course less cluttered just make your H5Ps Stealth activities 'Available but not shown on course page'. The grades will be in the H5P activity not the book.
We use the H5P plugin instead of the Content type so not sure how that would work.