From Nagios Support Wiki
Official information on Nagios XI can be found here.
Nagios XI is an enterprise-class monitoring solution that provides organizations with extended insight of their IT infrastructure before problems affect critical business processes.
Getting Started With XI
Find help on getting started with Nagios XI.
Video tutorials that guide you through various aspects of XI usage and concepts.
Looking to build for Nagios XI. Get information on the Nagios XI APIs here.