In case you missed it:
Download: https://www.couchbase.com/downloads
Click on pre-release versions at the bottom.
Overview of N1QL & GSI features in Couchbase 5.5:
What’s new in docs: https://developer.couchbase.com/documentation/server/5.5/introduction/whats-new.html
Blogs:
https://blog.couchbase.com/new-querying-features-couchbase-server/
https://blog.couchbase.com/ansi-join-support-n1ql/
(More on the way)