Yes. Nook is published open source (licensed under AGPL-3.0) and published at https://lab.civicrm.org/outlandish/nook
What you can and can’t do with the source code:
With AGPLv3, you can freely use, modify, and distribute software, even commercially, but you must share your source code if you offer it over a network (like a web service), ensuring others get the same freedom (strong copyleft). This includes making changes available and keeping licenses/copyrights intact.
It’s intended to prevent people from benefiting commercially by taking open-source code, customising it, and then charging customers for online services that use the underlying code whilst no longer sharing their customisation.
You can read more here: https://fossa.com/blog/open-source-software-licenses-101-agpl-license/