We already use CiviCRM. Can I still access its functionality if I switch to Nook?

Yes. Nook is built on top of CRM. If you already use “core CiviCRM” functionality such as emailing, or to record other kind of Contacts (such as “approved installers”), you can still access them in your CiviCRM setup. If you can’t already see a navigation item in the sidebar for “CiviCRM” talk to Outlandish / your Nook developer.