[jjirsa]

Datastax academy is probably the best free source

Searching YouTube for Cassandra summit talks is probably second

There was a push to do some better docs on the ASF website but it's just manpower that is currently spending time writing code instead - we have no real full time doc writers that focus on the open source product. Maybe some day someone will volunteer (and if you want to volunteer, I'll commit the docs for you - the site has a how to contribute guide, but honestly I'll take GitHub PRs if they're nontrivial even though it's an annoying workflow for our non-GitHub master).