change the url path for the book grid taxonomy

Unfortunately that is not possible at this time. You are able to change the URL in the Mooberry Book Manager Settings -> Book Grid page, but you cannot add a URL with a / in it.

Ok, thank you for your answer! Yes, I tried to change the book grid page, but that doesn’t affect the orther url paths.
The point is that on an author site, usually the books are displayed on a “books page” and I would want all grids and taxonomy pages to be subpages of the books page.
I have a problem with the links to genres and series on the single book page. A solution would be to rewrite these urls to a grid page of series, tags, etc. I made grids and separate pages for all genres and tags, so I know where to link to.
Could this be done with a filter that systematically replaces each tag and genre url with another specified url?

Eg on this page: https://caster.be/understrap/book/at-the-waters-edge/
The Genres link at the bottom of the page links to https://caster.be/understrap/genre/novels. My novels grid is at this location: https://caster.be/understrap/books/novels/

I think I could also solve this by making redirects in .htaccess, but maybe that’s not the best solution.

There is a filter mbdb_shortcode_{$permalink}_taxonomy where {$permalink} is whatever value you have set on the Book Grids setting page for the taxonomy URL slug. You could use that filter to search and replace the URL in the string.