https://jaceklaskowski.gitbooks.io/mastering-apache-spark/ https://pages.databricks.com/definitive-guide-spark.html
Databricks cloud workspace (community edition) offers free interactive spark clusters where you can practice or develop your skills https://community.cloud.databricks.com/
If you want a bit of flexibility, you can launch your own cluster using flinktrock and a pair of free tier t2.micro EC2 instances https://github.com/nchammas/flintrock