You may be able to demonstrate engagement through a range of other metrics.
Book metrics
There may be other metrics that you can use to measure book influence.
eBook metrics
Awards or prizes
Record any essay, book, researcher, presenter or other prizes related to your publications.
Invitations to speak
Record any invitations to speak at prestigious gatherings, conferences, or public events to address industry bodies, government etc.
Non-traditional research outputs
Record the impact of any non-traditional research outputs