The Nyzo verifier, sentinel, client, Micropay server, Micropay client, and documentation server are all distributed in the same source repository. It is available at https://github.com/n-y-z-o/nyzoVerifier.
The Nyzo white paper was modified once since the launch of the Nyzo blockchain. This modification is described in Nyzo white paper notes, #1. The white paper is still largely accurate and is a good primer on Nyzo. However, the documentation server, tech.nyzo.co, offers more substantial insight into how Nyzo works.
The bulk of Nyzo's documentation is in the release notes. These notes describe all changes made to the Nyzo code since the blockchain was launched, with the limited exceptions of incomplete release notes for version 475 and missing release notes for versions 476 and 477. Also, some release notes have not yet been ported to the documentation repository. These release notes can be found on this server, starting at version 478.