adding apache spark books

free-programming-books.md

@@ -164,6 +164,7 @@
 * [Scratch](#scratch)
 * [Sed](#sed)
 * [Smalltalk](#smalltalk)
+* [Spark](#spark)
 * [SQL (implementation agnostic)](#sql-implementation-agnostic)
 * [SQL Server](#sql-server)
 * [Standard ML](#standard-ml)
@@ -2006,6 +2007,13 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [Stef's Free Online Smalltalk Books](http://stephane.ducasse.free.fr/FreeBooks.html) (meta-list)
 
 
+### Spark
+
+* [Databricks Spark Knowledge Base](https://www.gitbook.com/book/databricks/databricks-spark-knowledge-base/details)
+* [Databricks Spark Reference Applications](https://www.gitbook.com/book/databricks/databricks-spark-reference-applications/details)
+* [Mastering Apache Spark](https://www.gitbook.com/book/jaceklaskowski/mastering-apache-spark/details)
+
+
 ### SQL (implementation agnostic)
 
 * [A Primer on SQL, Second Edition](https://leanpub.com/aprimeronsql)